IMO, targeting C89 is only warranted when you know for a fact it's needed. After all, not only is C99 a good improvement but toolchains stuck on C89 are not the type of toolchain I'd want to support.
Until somewhat recently C89 was the latest version of C that was supported by the MSVC compiler. Support for C99 was always terribly broken and Microsoft chose to claim it did supported C99 but without supporting mandatory features, which meant they did not in fact supported C99.
This sad state of affairs only changed significantly in 2020, with a lowkey announcement.
https://devblogs.microsoft.com/cppblog/c11-and-c17-standard-...
Likewise, even with the updated C11 and C17 support, it isn't 100% there, only good enough, because C++ is what matters for Windows developers when not using .NET.