If we look at the very last row of POSIX 2.2.2 The Name Space table, we see that "ANY header" has reserverved the suffix "_t":
That said, several MariaDB source defined types – for instance "my_ptrdiff_t" – are aliases for system types and while conflicting with POSIX technically, they do so in such a way which seems very likely to remain safe. Furthermore, migrating the names to something else would likely result in conveying less information to developers. Also, several of these are in use in a lot of connectors and plugins.
Many other conflicting types are new inventions which are not widely used outside of the server source – for instance "json_string_t" – and these run a slight risk of clashing with future POSIX types. In the cases where the use of the "_t" suffix does not convey important information to developers, it would be better to migrate to a naming convention which does not conflict with POSIX.