The Quake Champions beta is only accessible through the Bethesda.net launcher for the time being, but you will be able to play the game from Steam with a linked Bethesda.net account when the game properly releases later this year. The information comes from a recent developer inquiry posted on the Bethesda forum: something I found amusing is the suggestion that Capture the Flag and other classic multiplayer modes aren’t “an immediate priority” for the team, though I imagine they will show up eventually.
“Quake Champions will indeed be released on Steam. You will still be required to have a linked Bethesda.net account, but users playing the game from Steam will not go through the Bethesda.net launcher. During the current beta period the game will only be available through the Bethesda.net launcher.” At the very least, the Steam release should open it up to a wider player base and ensure that there’s enough people in the servers to get a decent game whenever you want.
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Source: [H]ardOCP – Quake Champions Will Be Available on Steam