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.
"The PC version of Fallout 76, for both the B.E.T.A. and the launch, will be available only via Bethesda.net, not on Steam," a Bethesda rep said in an email.
That comes as a real surprise: Bethesda would obviously like to push more widespread adoption of its own launcher, but foregoing the biggest, most popular PC digital distribution platform on the planet is a ballsy move. That's not to say it won't happen eventually, but it may be quite a wait if it happens at all—Fallout Shelter was released on Bethesda.net in July 2016, but didn't make it to Steam until March 2017.
Another interesting note in the beta FAQ is that if all goes according to plan the beta version will actually be the full game, and any progress you make in it will carry over into the release version.
"Our current plan for the B.E.T.A. is it will be the full game and all your progress is saved for launch," it states. The beta will also not be under NDA, although that was previously known—but it takes on an increased significance if the beta build is actually the full game.
The Fallout 76 beta test is set to begin sometime in October. Everyone who preorders the game will be invited to take part.