Maps with coop-only keys

Has there ever been any vanilla doom maps where the key(s) to beat the map only appear in coop mode? And I mean custom maps.

TNT MAP31

Not really related, but Eternal map28 has a multiplayer-only teleport destination that leads to a second exit.

scifista42 said:

TNT MAP31

Some clarification: custom maps.

You know, this would be a nice mapping technique. Add alternate routes through the level accessible via co-op only keys, maybe even block off part of the normal route with MP-only decorations to switch things up. Memento Mori did something like this with Map29 I believe.