Zandronum

Can anyone recommend me a WAD which isn't overly breakable with jumping, and supports co-op?

Besides Jump Maze?
But it all depends on what kind of jumping you mean, casual jumping, Rocket jumping?

I assume he means casual.

I know there's an Enjay wad (I can't remember which) that requires jumping, but no rocket jumping.

Casual, yes. IIRC Jumpmaze has very few enemies, I'd rather something with a normal monster density.

Any WAD which isn't overly breakable via jumping is ok.

Zpack, Zblood, Demon Eclipse (Well this requires GZdoom but older versions do work on Zandronum I heard), Stronghold, Delta Invasion (I know these two are for invasion but, they are similar to coop anyway :D) possibly more but I can't think of any at the moment.

Here's a tip though, if it's plutonia or tnt it's not what you are looking for. Also any of the vanilla based map packs like Alien Vendetta, Hell Revealed etc.

I don't mind if there's only a few spots that break.

I'm thinking Scythe might be acceptable, it's very speedrun-friendly as it is so it's not big loss if a map gets broken by jumping.

Jump-friendly wads? Try Doom 2: No rest for the living

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game with wad(NRFTL) will be twice as fun(and hardcore), if you disable respawn weapons, items, powerups and enable friendly fire