There’s tons of stuff like that. E.g. open source Doom ports that still require the .wad files of the game.

Recently stumbled upon a remake of Dark Forces, called The Force Engine. High resolution play enabled, save games, crosshair. These kinds of mods aim to make the games better but they cannot legally provide the assets for them.

That song slapped so hard it made me copy my floppy’s even harder!

Theres also the thing that there are different emulators for the Switch available, emudeck installs Ryujinx. It seems for some games performance can vary on the SteamDeck with different emulators, and maybe some games run better with something that is Yuzu-based, e.g. the Citron emulator. So if you are happy to experiment you could probably find a good solution for most games.