Darkest Dungeon, Black Reliquary, Pillars of Eternity, Cloudpunk, Pathfinder 2, Divinity 2 Original Sin, Slay the Spire, Witcher 1 and 3, Morrowind, Oblivion, Torment, to name just a few...
I'm using Vanilla, but AFAIK modded would work just as well. The key for playing old, pre-controller games meant for big monitors on the Deck are two features: Stream Input and Native Zoom. I always map one of the back buttons (usually) to Toggle Zoom.
Please post about your experience with modded Morrowind, I might want to try that too!
You should definitely use OpenMW instead of vanilla, it's not a mod but is instead a full engine rewrite. It runs natively on linux, has better performance, and a whole lot of other benefits:
Native support for macOS, Linux, and Windows
Improved physics and AI
Distant terrain
Save/Load dialogs organized by character
Quality of life UI improvements, such as being able to search for spells
Multiple quicksaves
World map adjusts automatically to fit new landmass from mods such as Tamriel Rebuilt
Support for up to 2147483646 loaded mods (up from 255 in the original Morrowind engine)
Since it was made from scratch, virtually no engine bugs from the original Morrowind
And much more
You can install it from the Discover store in desktop mode and then add it to steam, or alternatively you can use a tool like Protonup-qt (also in the discover store) to install Luxtorpeda, which is a tool for automatically launching supported games with rewritten engines. Once Luxtorpeda is installed you can open Morrowind steam properties in game mode, and check the "force specific compatibility tool" box and select to run the game with Luxtorpeda. After that it will automatically run the game through the OpenMW engine instead.