It's 100 bucks to put a game on steam. That's a bit much for a jam game. Yes it is a work around towards server hosting but it's a very expensive one.
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You don't actually need to pay the $100 or publish anything to use Steam multiplayer, that's the whole point of Spacewar (AppID 480). It's Valve's public test app, and you get Steam's networking + their relay for free, so P2P works without hosting your own servers. Just note it's meant for dev/testing not shipping (everyone shares the same lobby pool so you just make a filter for your own), and players need Steam open. Bit of a shady rep since pirates abuse it, but for jam co-op it's a free, legit way to get real online multiplayer.
Honestly I'd bet Valve would rather have jammers using it than pirates anyway. A jammer who makes something good might actually publish on Steam down the line, so it kind of funnels people toward them.