I certainly don't think we need to give exact history on the provenance of
each interface. A simple statement about current standards is good. But
that could give the erroneous impression that stpcpy was not available in
the GNU C Library before it supported POSIX.1-2008. (In fact, AFAICT
stpcpy has been in libc since before its first release.)
How about this?
This function is part of POSIX.1-2008 and later editions, but was
available in @theglibc{} and other systems as an extension long
before it was standardized.
I didn't say "GNU extension" to avoid the implication that we were its
inventors.