Comment by flohofwoe
Still debatable since the C standard doesn't reserve the _t postfix (it does reserve a single leading underscore followed by a capital letter, e.g. _Bool, and IIRC it also reserves two leading underscores).
What POSIX reserves or doesn't reserve doesn't affect code that follows only the C standard but doesn't care about POSIX compatibility, and especially _t is so widely used in C libraries that POSIX's opinion obviously doesn't matter all that much in the real world.
Whilst you do point to 6.4.3 for where it does reserve "All identifiers that begin with a double underscore (__) or begin with an underscore (_) followed by an uppercase letter"... That section also has the lovely:
> Other identifiers may be reserved.
If an implementation of C uses it... Just... Don't. The standard won't save you here, because it's happy for an implementation to do whatever they feel like.