Did anything ever come of QuestNet?

I would like to write an online game, that can be played as a single player but has live leaderboards and the possibility to have co-op quests for friends.

You mean QuestKit? It's a work in progress.

I think the OP means the experimental multiplayer Quest. As far as I know no one took it past the experimental stage, but it probably still works. It's in the forums here somewhere.

If you mean the old version of Quest that had multiplayer, I don't think that's supported anymore.