The dependencies might have a minimum C++ version, but I'd be pretty surprised if the core LuaTeX files couldn't be compiled with C99/C11. The recent C23 commits are just about adding support for compiling in C23 mode, so the main change is no longer using the implicit "everything is an int" function prototypes.
Thanks. The HB C++17 dependency is already a show-stopper for me. I want to be able to build everything on GCC 4.8, or even better 4.7. I also did some research what actually changed since 1.18.0 and didn't find features my project would depend on.