I would consider the union initializer change (require adding -fzero-init-padding-bits=unions for old behavior) much more hidden and dangerous, which is not directly related to ISO C23 standard.
I would count it as doing maintenance work for the upstream, kudos for doing this!
Huh. Wonder what the benefits of that are. Bools being ints never struck me as a serious problem. I wonder if this catches people who accidentally assign rather than compare tho....
The libraries you listed are all full of platform-specific code, and also have plenty of compiler-specific code behind ifdefs (for instance the stb headers have MSVC specific declspec declarations in them).
E.g. there is hardly any real-world C code out there that is 'pure standard C', if the code compiles on different compilers and for different target platforms then that's because the code specifically supports those compilers and target platforms.
I'd target the latest C standard and won't even care to know how many old, niche compilers I'm leaving out. These are vastly different uses for C and obviously your gaols drastically change your standard or compiler targeted.