The increase in bots and cheaters got progressively worse as matchmaking took over and ate away at the ability of private servers to maintain a community. Also before that, TF2 becoming a giant microtransaction experiment certainly didn't help.

Still though even today I can open up TF2 and still have a few favorited servers that have been going for a decade+. I can join those servers and play bot and cheater free for the most part.