The Fallout 76 beta FAQ states, among other things, that “the B.E.T.A. [that’s a Fallout-style acronym for Break it Early Test Application] and the game will be available on Xbox One, PlayStation 4, and on PC (via Bethesda.net only).” The Fallout 76 purchase page also bears only a generic “PC game” logo, with no mention of Steam. And today, Bethesda confirmed that neither the beta nor the full game will be released on the platform.
https://www.pcgamer.com/fallout-76-wont-launch-on-steam/