diff --git a/Lib/shutil.py b/Lib/shutil.py
index e29fe4d83e9277..55cfe35ab06977 100644
--- a/Lib/shutil.py
+++ b/Lib/shutil.py
@@ -40,6 +40,8 @@
     import nt
 
 COPY_BUFSIZE = 1024 * 1024 if _WINDOWS else 64 * 1024
+# This should never be removed, see rationale in:
+# https://bugs.python.org/issue43743#msg393429
 _USE_CP_SENDFILE = hasattr(os, "sendfile") and sys.platform.startswith("linux")
 _HAS_FCOPYFILE = posix and hasattr(posix, "_fcopyfile")  # macOS