Comment by shakna
The C23 spec also says:
> A potentially reserved identifier becomes a reserved identifier when an implementation begins using it or a future standard reserves it, but is otherwise available for use by the programmer.
Which, in practice, does mean using _t is likely to cause you problems, as it may become a reserved identifier, when an implementation like POSIX begins using it.
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.