Thanks to OpenMW, my SteamDeck has become a "Morrowind machine" -- it runs so well and it's so convenient to have all that world with me on the go. You can use https://luxtorpeda.gitlab.io which registers as one of the runtimes Steam uses to launch games -- Proton being one of them -- and behind the scenes Luxtorpeda will download and run OpenMW instead of Morrowind.exe.