The _pendwho set was removed because _pendwho is never set True anywhere.  (This may be an entirely separate bug/discussion.)

I didn't feel this check was worth adding yet another conditional (as it could only ever work if _tracking_channels was True); the only real way it could happen is if the channel was parted before its WHOX timer fired.  Actually, that may be a useful reason to use _pendwho: to kill the timer if the channel is parted before it's fired.