Really?! I'm actually kinda surprised, I would have guessed it's a much more recent abbreviation than that. I think I only started seeing it around 2010 or so.
"tl;dr" was around when I hung out on LiveJournal, so definitely pre-2010. This would have been around '01-'06.
Anecdotally I remember first encountering it on the Something Awful forums. I always assumed it originated there (like lots of stuff did in the early 2000s).