I managed to get Steam installed on my OnePlus 6T and Xiaomi Pad 5 Pro, both running postmarketOS, using Distrobox to create an Ubuntu 24.04 container and then installing FEX-Emu inside of it. I wrote up a guide on the postmarketOS wiki on how to do it, some issues I ran into, some tips on how to get around those issues, and a list of games I've tested. Feel free to expand upon this list if you try it out. Older games such as Half Life 2 are quite playable, especially if your device supports keyboard and mouse input. I have not yet tested using a controller.