We call our lib Ain Soph. It's never been publically released (though that certainly hasn't stopped other MUDs from being started using it). The only thing it was ever based on was stock 2.4.5, and of course there's no trace of that left.
I voted "I don't use an LPMud," because my MUD is written in Python. :-) But it still uses the concept of a "mudlib," by separating game functionality from server functionality. You could call the game functionality the "mudlib," because it doesn't know any details about what the server portion does – it just relies on certain functions being made available by the server. Thus, the server implementation can change, as long as it exposes the same interface and semantics, and the game won't care.