glibc changes like this are fine

3. Hyrum's law. Most programmers do not read the C specification, they read (at most) the man page, which (on my WSL installation of Debian), says that realloc(ptr, 0) is equivalent to malloc(0) if ptr == NULL and free(ptr) otherwise (the return value section does mention that realloc(ptr, 0) may return a non-NULL pointer, but it does not distinguish between the ptr == NULL case and the ptr != NULL case, so the most straightforward interpretation is that it is talking about the former). There is no mention of portability anywhere in the document that I could find (the document claims that realloc is "conforming to" various versions of both the POSIX and C standards, without any note of this extension). Some programmers will not even go that far, they just write some code and test if it works as expected. Then the code silently picks up a glibc dependency that nobody explicitly knows about. This is not hypothetical - there is at least one comment in this thread from a developer who actually did take such a dependency and had to patch their code for musl compatibility.

In case anyone is wondering whether it is up-to-date: I honestly have no idea. It is probably outdated, because Debian, but I do not know off the top of my head how to confirm that. The colophon says it's from Linux man-pages version 5.10 and has a URL pointing to https://www.kernel.org/doc/man-pages/, which in turn tells me nothing at all about the current version of that project.