When the EVENTHANDLER(9) subsystem was created, it was a documented feature that an eventhandler callback function could safely deregister itself. In r200652 that feature was inadvertantly broken by adding drain-wait logic to eventhandler_deregister(), so that it would be safe to unload a module upon return from deregistering its event handlers.
There are now 145 callers of EVENTHANDLER_DEREGISTER(), and it's likely many of them are depending on the drain-wait logic that has been in place for 8 years. So instead of creating a separate eventhandler_drain() and adding it to some or all of those 145 call sites, this creates a separate eventhandler_drain_nowait() function for the specific purpose of deregistering a callback from within the running callback.