That’s even worse though. Plenty of games (e.g. Stellaris and RimWorld) are also available on platforms like GOG or, ugh, Epic. But if you want to use mods and you bought the game on any platform other than Steam it’s fuck you.
I know there are workarounds, but this is true. There are very little games I buy (at least directly) through steam nowadays, because I didn’t like what it became after the Greenlight/Direct debacle and I didn’t want my library to be that dependent of them anymore.
I have playnite as a unified game library launcher (with GoG, itch.io, humble, Ubi, EA, even Amazon Prime and freaking EGS just for the free games), so where I get my games from doesn’t matter much for me now.
But workshop integration is basically the only thing that makes me want a Steam copy for a game.
Though among the games in that case, there were Rimworld and Dwarf Fortress, and for both if you get a copy directly from the developers, you get DRM-free and a Steam key. So, that’s what I did.
Even if you buy Terraria off of steam, you can use steam mods. Sounds like a per-game problem, rather than a steam problem.
https://steamworkshopdownloader.io/ never gotten this to work myself but I put the least amount of effort in as possible. There may be others as well but I remember when I did my research a couple years ago that it was a real trudge and almost not worth it.