> There's the old and new C runtimes in MSVCRT.DLL

Some advertisement for VC-LTL: https://github.com/Chuyu-Team/VC-LTL5 and https://github.com/Chuyu-Team/YY-Thunks

VC-LTL Implements a good subset of modern CRT functions using the msvcrt.dll, which makes modern MSVC compiler to emit binaries that theoretically can run in Windows NT 4.1 or Windows 98 (until CFG broke them, but you can turn it off). Still can teach an old dog new tricks.

YY-Thunk is a little more funky, it tries to implement new Win32 userspace API for old system by emulating the behavior and expected values using userspace API in the old apartment, so you can try to implement a function in Windows 10 with functions in Windows Vista, and hopefully it will work out.