Steam Group Chats are sort of there; no video chat but text chats and drop-in voice chats like Discord. On the other hand they're basically ephemeral, with messages disappearing from history at some given point.

I also can't figure out a way to access them outside of the Steam client and in DOTA where I believe they're tied to the in-game guild system.

Outside of Dota, it's called "Group Chats" (below your friends list) and it looks very similar to a basic Discord interface. You may have to join a dota guild in order to see it, although everyone in my guild just uses discord.

Steam itself is proprietary, and I imagine they'd expand the existing Steam chat and not do something separate like Proton. I don't think jumping into the arms of another company managing a centralized proprietary social platform is a good idea, even if Valve tend to be "good guys".