Valve are definitely up to something. For a little while, Valve developer Pierre-Loup Griffais has been tweaking steamcompmgr, the SteamOS session compositing window manager.
After being quiet on SteamOS development for a long time with no update since July last year, it certainly seems now like some parts of it are being revived either for the next major SteamOS release or Valve's other Linux gaming projects. Work on steamcompmgr seemed to stall back in 2018, with it suddenly seeing activity on GitHub in October last year.
In fact, it's no longer named steamcompmgr and seems to be expanding to do a whole lot more. It's had so many tweaks and changes, Griffais has actually given it a new name. Meet Gamescope (GitHub), which Griffais said when renaming it that "We're a superset of steamcompmgr now, but have a wider scope, so new name to reflect it.".
Going over the project there's a few things that stick out. It seems that they're going with Vulkan and Wayland (what's supposed to eventually replace Xorg in most major Linux distributions). Exciting!
Once they talk more about what they're doing and their plans, we will let you know. With all that Valve's doing including Steam Play, the Linux container system, this Gamescope and various other projects they're certainly still giving Linux gaming plenty of attention.