The requiresUnblock method on an action returns true if a user can be blocked from the action; otherwise it returns false.
The default implementation, on Action, returns true. Some subclasses of Action override it to return false. Some subclasses of these subclasses should re-override it to return true, but don't (example in T210953).
This was not a problem until T208862, which checks requiresUnblock in Title::getUserBlock, meaning that it is important that descendants of Action return the correct result.
Before we can go ahead with T208862, we need to do an audit of all subclasses of (subclasses of Action for which requiresUnblock returns false), and possibly correct some of them.