diff --git a/README/ReleaseNotes/v642/index.md b/README/ReleaseNotes/v642/index.md
index 8eb3d7de6b291..33ed73e02558c 100644
--- a/README/ReleaseNotes/v642/index.md
+++ b/README/ReleaseNotes/v642/index.md
@@ -66,8 +66,9 @@ Users are encouraged to export their models to ONNX and use the retained ONNX pa
 * The **RooStats::DebuggingSampler** and **RooStats::DebuggingTestStat** classes are removed. They were mock implementations of the `TestStatSampler` and `TestStatistic` interfaces that returned uniform random numbers independent of the data, only meant for debugging the RooStats framework itself during its initial development.
 * The `RooTrace` class is deprecated and will be removed in ROOT 6.44. It was a RooFit-specific memory tracer whose instrumentation hooks are compiled out by default, so it has been inert and untested for years. For memory debugging, please use general-purpose tools such as AddressSanitizer or Valgrind instead.
 * Support for the AIX operating system has been removed from the codebase. This support has not been tested since the late v5 releases and the LLVM JIT is not yet supporting AIX.
-* The headers Htypes.h and Gtypes.h that were deprecated in ROOT 6.20 will now emit warnings and will be fully removed in ROOT 6.44.
-* The header GLConstants.h is no longer part of ROOT installed headers.
+* The headers `Htypes.h` and `Gtypes.h` that were deprecated in ROOT 6.20 will now emit warnings and will be fully removed in ROOT 6.44. Use `Rtypes.h` as replacement.
+* The header `GLConstants.h` is no longer part of ROOT installed headers.
+* The header `Strlen.h` is no longer part of installed ROOT headers. Use `<cstring>` directly as a replacement.
 
 ## Build System
 
diff --git a/core/base/inc/Strlen.h b/core/base/inc/Strlen.h
deleted file mode 100644
index 7f1cb19a56549..0000000000000
--- a/core/base/inc/Strlen.h
+++ /dev/null
@@ -1,20 +0,0 @@
-/* @(#)root/base:$Id$ */
-
-/*************************************************************************
- * Copyright (C) 1995-2000, Rene Brun and Fons Rademakers.               *
- * All rights reserved.                                                  *
- *                                                                       *
- * For the licensing terms see $ROOTSYS/LICENSE.                         *
- * For the list of contributors see $ROOTSYS/README/CREDITS.             *
- *************************************************************************/
-
-#ifndef ROOT_Strlen
-#define ROOT_Strlen
-
-#include <ROOT/RConfig.hxx>
-
-#ifdef NEED_STRING
-#include <cstring>
-#endif
-
-#endif
diff --git a/core/base/src/TAttPad.cxx b/core/base/src/TAttPad.cxx
index dcc9a529cfc8b..35e9a6b38aa90 100644
--- a/core/base/src/TAttPad.cxx
+++ b/core/base/src/TAttPad.cxx
@@ -9,11 +9,11 @@
  * For the list of contributors see $ROOTSYS/README/CREDITS.             *
  *************************************************************************/
 
-#include "Strlen.h"
 #include "TAttPad.h"
 #include "TBuffer.h"
 #include "TStyle.h"
 
+#include <cstring>
 
 /** \class TAttPad
 \ingroup Base
diff --git a/core/base/src/TAttText.cxx b/core/base/src/TAttText.cxx
index d5bbc07fe8234..8d0ccac2e5d0a 100644
--- a/core/base/src/TAttText.cxx
+++ b/core/base/src/TAttText.cxx
@@ -9,8 +9,6 @@
  * For the list of contributors see $ROOTSYS/README/CREDITS.             *
  *************************************************************************/
 
-#include <iostream>
-#include "Strlen.h"
 #include "TROOT.h"
 #include "TAttText.h"
 #include "TVirtualPad.h"
@@ -21,6 +19,8 @@
 #include "TError.h"
 #include "TColor.h"
 
+#include <iostream>
+#include <cstring>
 
 /** \class TAttText
 \ingroup Base
diff --git a/core/base/src/TDatime.cxx b/core/base/src/TDatime.cxx
index fdb38429fcc0d..15750f16997c0 100644
--- a/core/base/src/TDatime.cxx
+++ b/core/base/src/TDatime.cxx
@@ -26,22 +26,19 @@ required, use TTimeStamp.
 
 #include <ROOT/RConfig.hxx>
 
-#include <ctime>
-
 #ifdef WIN32
 #include "Windows4Root.h"
-#include <cstring>
 #endif
 
 #include "TBuffer.h"
-#include "Strlen.h"
 #include "snprintf.h"
 #include "TDatime.h"
 #include "TError.h"
 #include "Bytes.h"
 #include "TString.h"
 
-
+#include <ctime>
+#include <cstring>
 
 ////////////////////////////////////////////////////////////////////////////////
 /// Create a TDatime and set it to the current time.
diff --git a/core/base/src/TDirectory.cxx b/core/base/src/TDirectory.cxx
index 136a7cb5b7ff2..28bbe2fdadce5 100644
--- a/core/base/src/TDirectory.cxx
+++ b/core/base/src/TDirectory.cxx
@@ -8,9 +8,6 @@
  * For the licensing terms see $ROOTSYS/LICENSE.                         *
  * For the list of contributors see $ROOTSYS/README/CREDITS.             *
  *************************************************************************/
-#include <cstdlib>
-
-#include "Strlen.h"
 #include "strlcpy.h"
 #include "TDirectory.h"
 #include "TBuffer.h"
@@ -31,6 +28,8 @@
 
 #include <algorithm>
 #include <limits>
+#include <cstdlib>
+#include <cstring>
 
 const Int_t  kMaxLen = 2048;
 
diff --git a/core/base/src/TFolder.cxx b/core/base/src/TFolder.cxx
index 7fa2dc84f011a..d59187e999a01 100644
--- a/core/base/src/TFolder.cxx
+++ b/core/base/src/TFolder.cxx
@@ -84,8 +84,6 @@ to save the pointer to the object as a class member or local variable
 if this pointer is used frequently or inside loops.
 */
 
-#include <iostream>
-#include "Strlen.h"
 #include "strlcpy.h"
 #include "TFolder.h"
 #include "TBrowser.h"
@@ -95,6 +93,9 @@ if this pointer is used frequently or inside loops.
 #include "TError.h"
 #include "TRegexp.h"
 
+#include <iostream>
+#include <cstring>
+
 static const char *gFolderD[64];
 static Int_t gFolderLevel = -1;
 static char  gFolderPath[512];
diff --git a/core/base/src/TNamed.cxx b/core/base/src/TNamed.cxx
index 71724c5bf74b3..f6603eb045dbf 100644
--- a/core/base/src/TNamed.cxx
+++ b/core/base/src/TNamed.cxx
@@ -20,13 +20,13 @@ Most member functions defined in this base class are in general
 overridden by the derived classes.
 */
 
-#include <iostream>
-#include "Strlen.h"
 #include "TNamed.h"
 #include "TROOT.h"
 #include "TVirtualPad.h"
 #include "TClass.h"
 
+#include <iostream>
+#include <cstring>
 
 ////////////////////////////////////////////////////////////////////////////////
 /// TNamed copy ctor.
diff --git a/core/foundation/inc/ROOT/RConfig.hxx b/core/foundation/inc/ROOT/RConfig.hxx
index 6f49eedae02f9..d11da3bb0c933 100644
--- a/core/foundation/inc/ROOT/RConfig.hxx
+++ b/core/foundation/inc/ROOT/RConfig.hxx
@@ -99,7 +99,6 @@
 #      include <cstdlib>
 #   endif
 #   define R__UNIX
-#   define NEED_STRING
 #   define NEED_SIGJMP
 #   if __SUNPRO_CC > 0x420
 #      define R__SOLARIS_CC50
@@ -117,7 +116,6 @@
 #   define R__SEEK64
 #   define ANSICPP
 #   define R__UNIX
-#   define NEED_STRING
 #   define NEED_SIGJMP
 #endif
 
@@ -384,7 +382,6 @@
 #   define SC
 #   define R__SC
 #   if defined(WIN32)
-#      define NEED_STRING
 #      define NEED_STRCASECMP
 #      define NEED_SNPRINTF
 #      define ANSICPP
@@ -397,7 +394,6 @@
 
 #ifdef _MSC_VER
 #   define R__VISUAL_CPLUSPLUS
-#   define NEED_STRING
 #   define NEED_STRCASECMP
 #   if _MSC_VER < 1900
 #     define NEED_SNPRINTF
diff --git a/core/meta/src/TDataMember.cxx b/core/meta/src/TDataMember.cxx
index 63d3801a40ff7..871ef9c07c42c 100644
--- a/core/meta/src/TDataMember.cxx
+++ b/core/meta/src/TDataMember.cxx
@@ -137,7 +137,6 @@ but also in Dump() and Inspect() methods and by the THtml class.
 
 #include "TDataMember.h"
 
-#include "Strlen.h"
 #include "strtok.h"
 #include "strlcpy.h"
 #include "TBuffer.h"
@@ -160,7 +159,7 @@ but also in Dump() and Inspect() methods and by the THtml class.
 #include <cassert>
 #include <cctype>
 #include <cstdlib>
-
+#include <cstring>
 
 
 ////////////////////////////////////////////////////////////////////////////////
diff --git a/core/meta/src/TFunction.cxx b/core/meta/src/TFunction.cxx
index bcf6a98a32d0c..544e623df340e 100644
--- a/core/meta/src/TFunction.cxx
+++ b/core/meta/src/TFunction.cxx
@@ -21,11 +21,11 @@ functions (accessible via TROOT::GetListOfGlobalFunctions()).
 #include "TList.h"
 #include "TROOT.h"
 #include "TInterpreter.h"
-#include "Strlen.h"
 
-#include <iostream>
 #include "TVirtualMutex.h"
 
+#include <cstring>
+#include <iostream>
 
 ////////////////////////////////////////////////////////////////////////////////
 /// Default TFunction ctor. TFunctions are constructed in TROOT via
diff --git a/core/meta/src/TMethod.cxx b/core/meta/src/TMethod.cxx
index e57d69ed28db4..e9260cc640a9e 100644
--- a/core/meta/src/TMethod.cxx
+++ b/core/meta/src/TMethod.cxx
@@ -27,10 +27,9 @@
 #include "TMethodArg.h"
 #include "TMethodCall.h"
 #include "TInterpreter.h"
-#include "Strlen.h"
 #include "TDataMember.h"
 
-
+#include <cstring>
 
 ////////////////////////////////////////////////////////////////////////////////
 /// Default TMethod ctor. TMethods are constructed in TClass.
diff --git a/core/meta/src/TMethodArg.cxx b/core/meta/src/TMethodArg.cxx
index 1b75367408c87..8f1f59fc2ff6f 100644
--- a/core/meta/src/TMethodArg.cxx
+++ b/core/meta/src/TMethodArg.cxx
@@ -11,11 +11,12 @@
 
 #include "TMethodArg.h"
 #include "TInterpreter.h"
-#include "Strlen.h"
 #include "TMethod.h"
 #include "TMethodCall.h"
 #include "TDataMember.h"
 
+#include <cstring>
+
 /** \class TMethodArg
 Each ROOT method (see TMethod) has a linked list of its arguments.
 This class describes one single method argument.
diff --git a/core/meta/src/TMethodCall.cxx b/core/meta/src/TMethodCall.cxx
index e7188d1d34e88..a49508fb4e763 100644
--- a/core/meta/src/TMethodCall.cxx
+++ b/core/meta/src/TMethodCall.cxx
@@ -27,10 +27,10 @@ occur if parameters involve temporary object construction.
 #include "TMethod.h"
 #include "TClass.h"
 #include "TROOT.h"
-#include "Strlen.h"
 #include "TVirtualMutex.h"
 #include "TError.h"
 
+#include <cstring>
 
 ////////////////////////////////////////////////////////////////////////////////
 /// Default TMethodCall ctor. Use Init() to initialize the method call
diff --git a/graf2d/gpad/src/TAttCanvas.cxx b/graf2d/gpad/src/TAttCanvas.cxx
index ea1566a73215e..790f6358acd27 100644
--- a/graf2d/gpad/src/TAttCanvas.cxx
+++ b/graf2d/gpad/src/TAttCanvas.cxx
@@ -9,9 +9,8 @@
  * For the list of contributors see $ROOTSYS/README/CREDITS.             *
  *************************************************************************/
 
-#include "Strlen.h"
 #include "TAttCanvas.h"
-
+#include <cstring>
 
 /** \class TAttCanvas
 \ingroup gpad
diff --git a/graf2d/graf/src/TWbox.cxx b/graf2d/graf/src/TWbox.cxx
index a2fb8b9bd8619..ed74c9274a88f 100644
--- a/graf2d/graf/src/TWbox.cxx
+++ b/graf2d/graf/src/TWbox.cxx
@@ -9,15 +9,15 @@
  * For the list of contributors see $ROOTSYS/README/CREDITS.             *
  *************************************************************************/
 
-#include <iostream>
 #include "TROOT.h"
-#include "Strlen.h"
 #include "TWbox.h"
 #include "TColor.h"
 #include "TStyle.h"
 #include "TVirtualPad.h"
 #include "TVirtualPadPainter.h"
 
+#include <iostream>
+#include <cstring>
 
 /** \class TWbox
 \ingroup BasicGraphics
diff --git a/io/io/src/TDirectoryFile.cxx b/io/io/src/TDirectoryFile.cxx
index 2f598d32296a7..14ae6a2fb7896 100644
--- a/io/io/src/TDirectoryFile.cxx
+++ b/io/io/src/TDirectoryFile.cxx
@@ -23,8 +23,6 @@ End_Macro
  The structure of a file is shown in TFile::TFile
 */
 
-#include <iostream>
-#include "Strlen.h"
 #include "strlcpy.h"
 #include "TDirectoryFile.h"
 #include "TFile.h"
@@ -49,6 +47,9 @@ End_Macro
 #include "TVirtualMutex.h"
 #include "TEmulatedCollectionProxy.h"
 
+#include <iostream>
+#include <cstring>
+
 const UInt_t kIsBigFile = BIT(16);
 const Int_t  kMaxLen = 2048;
 
diff --git a/io/io/src/TFile.cxx b/io/io/src/TFile.cxx
index d3450dc3ee791..082363009a481 100644
--- a/io/io/src/TFile.cxx
+++ b/io/io/src/TFile.cxx
@@ -127,7 +127,6 @@ The structure of a directory is shown in TDirectoryFile::TDirectoryFile
 #include "Bytes.h"
 #include "Compression.h"
 #include "RConfigure.h"
-#include "Strlen.h"
 #include "strlcpy.h"
 #include "snprintf.h"
 #include "TArrayC.h"
@@ -172,8 +171,10 @@ The structure of a directory is shown in TDirectoryFile::TDirectoryFile
 #include "TGlobal.h"
 #include "ROOT/RConcurrentHashColl.hxx"
 #include "ROOT/InternalIOUtils.hxx"
+
 #include <memory>
 #include <cinttypes>
+#include <cstring>
 
 #ifdef R__FBSD
 #include <sys/extattr.h>