For (2) I guess it depends. Annex K is obviously already a part of the standard so it depends on the implementation. There is a push to eliminate Annex K altogether from the C Standard. If this push fails, it may be the case that more libraries will add support for this optional feature of the language. In the meanwhile, there is the Open Watcom compiler implementation [1], the Safe C Library [2], and Slibc [3].

[1] Watcom C Library Reference Version 1.8. Open Watcom. 2008. ftp://ftp.openwatcom.org/manuals/current/clib.pdf

[2] Safe C Library — A full implementation of Annex K https://github.com/rurban/safeclib/

[3] slibc https://code.google.com/archive/p/slibc/