This news was revealed via the Fallout 76 betaFAQ, which states that "the beta and the full game will be available on Xbox One, PlayStation 4 and on PC (via Bethesda.net only)".
Bethesda joins other publishers in making its next, hotly anticipated game available outside of Steam. However, PC players will have to use Bethesda's own platform if they want to womp cazadors with their mates in West Virginia. Some had assumed this would be the case, seeing as the Steam logo didn't appear anywhere on the site, and no mentions of the platform were made in the FAQ itself.
Apart from the original Fallout trilogy, Fallout games have typically been released on Steam - and why wouldn't they?
The Fallout 76 makers said that the: "Current plan for the B.E.T.A.is it will be the full game and all your progress is saved for launch". If you pre-ordered through a retailer, you'll need to head to Bethesda.net, create an account, and redeem your code - this will either be on your pre-order receipt or email confirmation.
For the first time in the modern-history of the game series, the next Fallout game, Fallout 76, will not launch on Steam when it's released sometime during October.