r86041 deprecated the UnknownAction hook with a comment in Action.php.
There is no mention of the deprecation in Wiki.php which is the first place that I think most people would look for the deprecation notice.
Also the hook run code does not issue a deprecation warning, it's not marked deprecated in doc/hooks.txt, sample code still uses it, ...
Extensions still using this hook (as of 2017-01-01): (No WMF-deployed extensions)
- examples/ContentAction (this one needs addressing)