The fact that one of the biggest contributing game development houses out there to date felt that it would be necessary to make an announcement that they would be indeed sticking around with Steam is a big surprise. According to a Tweet from Bethesda all their forth coming titles along with their November 2018 release Fallout 76 would be coming to the Steam platform.
Earlier this year Epic snatched away Metro Exodus from Steam and made it an Epic Store exclusive and most recently just added Control and The Outer Worlds to their future releases as well. The fact that Epic are now in talks with Ubisoft currently about future game releases gives rise to the most bold Tweet from Bethesda regarding their standing and future relationship with Steam.
This may also indicate that Bethesda is not necessarily ready to go head to head with their own platform against Steam's platform just yet, and are rather making the strategic decision to also state that Fallout 76 was not going to remain an exclusive to their platform either.
Bethesda have not officially announced whether or not these titles will launch later on Steam, or at the same time as on Bethesda's own platform. There also has been no mention of launching on other storefronts. How long the battle of the platforms will be endured no one really knows but Bethesda and Steam's mutual understanding has been presented moving forward for now.