Here's a simple criterion I propose: is the platform disbanded?

For example, in GNU Make's dir.c file. There's a lot of stuff like this:

    #ifndef _AMIGA
        return dir_file_exists_p (".", name);
    #else /* !AMIGA */
        return dir_file_exists_p ("", name);
    #endif /* AMIGA */

There should be a foreseeable date when we can say, "OK the Amiga maintainers have added a feature that lets us use '.' as a directory so we can now delete that #ifdef". But that day is guaranteed to never come, because the Amiga project is disbanded. So should we keep that until the heat death of the universe?

I would propose that we instead say, if you use Amiga, there's great support for it in versions of GNU Make up until x.y.z. So if you love old Amigas, you'll be well served using an older version of GNU Make. I think it's straightforward.