Cool project. I would relax the requirements from C99 to C89. You get more portability to cool
retro systems that way and C99 doesn’t really add that much. Also C++ compilers are generally more able to compile C89 in C++ mode than C99, e.g. msvc
Also it's 2020, I wasn't even coding when C99 came out and I'm now a "senior" developer, whatever that means. You'll have to pry C99 from my cold, dead hands. Whatever compilers don't support it by now, I don't want to support them.
I haven't programmed in C in over 10 years though, so I could be missing something.
Using C99 + POSIX is the defining fact about your project.
I would let somebody else write a C89 irc client, instead.
Also MSVC's C99 support is pretty good since ca. VS2015.
Naturally like everyone else in the C world, with exception of gcc/clang, all C99 features that became optional in C11 aren't planned to be supported.
But yeah, those C99 features weren't consistent at all with Herb Sutter's 2012 blog post about MSVC only supporting C features that are needed for the C++ compiler.
C and C++ are more strictly separated in the Microsoft compilers compared to gcc and clang (which both support more modern C features in C++ mode via non-standard extensions), I think that's what's confusing many people. It's not a problem in mixed-language projects though, just put all the C code into .c files and all C++ code into .cpp and you're set, compiling C code with a C++ compiler isn't such a great idea anyway, since it limits you to a ca. 1995 version of C.
Could just as well ask why not write it in Nim, Zig, Rust, Swift, Kotlin etc... This means a different audience, different target platforms, different trade-offs.