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.