Fastidious There should be info on mention. Post ID XXXX was deleted. In system even if it's content is removed or hidden from public, should stay info about it. Or other hand, if post just didn't exist in DB, so mention should say what 404 say: Never existed or deleted. Or just ignore mentioned post reference, just leaving username reference. If user is removed there should be as @Franz said.
Look here: Mention dummyuser to reply #2000 @dummyuser#2000 result blank hover toolltip. There should be info, that user or reply didn't exist. But I think if post was existed before should stay in DB just to information purposes. To indicate users it was existed when someone reply to it. But it was deleted, by user or admin/moderator. Info about WHO removed it is also nice. If for example: "Moderator removed this post because of forum rules violation"
