Update (Wed, 27th April, 2022 16:40 BST): Bethesda's migration process is now open! If you wish to move your Bethesda library over to Steam, the publisher has detailed the process in a new blog post.
From early April, users will be able to migrate their games and any existing digital currency over to Steam. And for games that require it, users will still have to use their Bethesda.net login to sign in and play.
In the same announcement, Bethesda also made it clear how these accounts would continue to be accessible on Bethesda's website, in-game, and would continue to be supported in future titles.
If you want to learn more about this sunsetting process, there's a detailed FAQ over on Bethesda's website.