GOG has reportedly cut dozens of jobs recently. Here are new details about the situation at CD Projekt’s subsidiary and the shortcomings of its business strategy.
As great as Gaben and Valve are his word doesn't mean anything (it's not a personal attack either, it's just that it isn't anything binding). Luckily if they fail to keep their promises (or legally cannot) then the crack community will step in. I'm pretty sure they cracked Steam DRM ages ago. I remember a friend using it for Left 4 Dead back in the day.
But the APIs are public, so they can be reimplemented in open source. There just hasn't been any reason for it since currently that would only be used for piracy (in fact some "cracked" games have a mockup of the steam API that just returns the expected things as if it had contacted the servers). But the moment steam goes away I give it a couple of weeks until there's a GitHub implementing most of the basic stuff.
I thought I had it running without starting steam before... That was pre-1.4 and the windows version though, so things may have changed. I know there used to be a wrapper that would start the game outside of steam, but that was ages ago.
I don't think this is because of multiplayer though, as you can just not use steam services for multiplayer and connect directly to IPs. In my case trying to run without steam started causing crashes with windows forms, so steam linux runtime is probably being used for at least a few things.
Copying terraria to a windows VM (which was far more work that it needed to be) results in something similar, with a TypeInitializationException, so Steam is needed for what looks to be some social API, maybe for grabbing social links? It's quite possible that there are more things, but I don't think Terraria requires steam multiplayer services, especially as the GOG version runs without steam.
I don't think that counts as DRM, but the end effect is similar: steam needs to be running. If steam ever dies, I'm certain a simple wrapper will be made to run Terraria and probably many similarly integrated games, but it is not ideal.