Since around Visual Studio 2015 you're fine.
AFAIK the STB headers are on C89 mainly because nothings likes to use VC6 as C IDE.
E.g. if you depend on other C99 features anyway, including stdint.h is fine, also on MSVC.
PS: FWIW I use stdint.h types in the Sokol headers since around 2017 and never heard complaints from users, with the supported compilers being MSVC, GCC and Clang.