|MetaFilter's site and server can always use upgrades of hardware, software, and bandwidth, as well as more stable funding for continued support of its small but high-skilled moderation and backend team! If you'd like to chip in, you can donate to Metafilter.|
These are threads that aren't yet Deleted Threads, but are clearly going to be deleted in the very near future (only moderator s can delete threads or comments). Usually these threads will have violated some of the requirements for posts. Typical offences include Double Posts, posts that are filled with profanity or garbage, and posts that are blatantly adverts.
These threads should never be touched. The main reason posting In Jokes and garbage in dead threads is bad is because it isn't anyone's place other than the moderators' to declare a thread dead. If everyone took to posting crap in threads they thought should be deleted, many threads would be wrecked. Put another way: you and I would probably disagree whether a particular thread should be deleted. You would likely be ticked if I began posting "pancakes" and the like to a thread you thought was worthwhile.
In practice, however, obviously dead threads do tend to fill up with various silliness.