I hope I get to use C2x* before I retire.
*postmodern C?
AFAIK the only non-supported C99 features in the C compiler are VLAs (optional since C11 anyway), and type-generic macros (those would be good to have though).
Of course it would be nice if Microsoft gave the C compiler a bit more love, especially since it's much less work keeping a C compiler uptodate than a C++ compiler, but at least we got "most of C99".
Although inline was added in C99, it was already an extremely widely supported extension in mainstream compilers, even since the C89 days, when we just called it "ANSI C".
MSVC has supported inline for a long time, long before it started supporting other C99 features.
I could understand the concern if it was about portability to other target platforms, or keeping the option of doing so. But in that case, the public standard your current target supports is irrelevant.
So they add a feature not supported by MSVC and don't learn that it doesn't work until someone else tries to build on Windows.
If you choose to use features based on whether they work or not, you don't need to choose a standard at all. But that loses you all of the guarantees a standard provides.