The title and message for this rule are phrased as though the rule will NOT be flagged on the file in question. E.G. "Add the missing header *in file XXX*" rather than "Add the missing header to *this file*" ...?
Note that some languages marked obsolete purely because the 'convention' tag was removed from this rule. If that tag had not yet been applied in the language, then the language isn't actually out of date.
This was a bit of a compromise [~evgeny.mandrikov], but it comes down to the fact that the default severity is Blocker and it's a bit difficult to justify rating a "convention" above Minor.