The previous requirement was that tools should only use C89 features.
Several tools did not follow this rule for years. Since nobody
complained about not being able to build NetBSD in a C89 environment,
drop that rule, now requiring C99.
https://mail-index.netbsd.org/tech-toolchain/2022/01/30/msg004091.html
It's not practical for the C89 restriction to be maintained for
compilers, but it is still desirable for most tools. The "long long"
data type is in wide use despite not being in C89. C99 library features
(as opposed to language features) can often be added to the compat
framework when the need becomes apparent.
rillig