estirose

Okay, your main choice on the Steamdeck (which is not mine, but it's passable) for Intellivision emulation is Retroarch. In fact, I think it's the only emulator that comes in flatpak/via Steam that you can use, and luckily, the Steam version just added freeintv as a core!

Most of the controls can be left at the Retroarch base config. The one adjustment you'll want to make is to invert the Y access (bottom of the joystick controls) for the right joystick. Why? Because the layout for the Intellivision keypad has the 1 button on the top left, while the mapping for Retroarch seems to be based on the 1 being in the bottom left. (ETA: This may have been fixed so that you no longer have to invert the Y axis to get your games to move correctly.)

The only thing I haven't figured out is how to press the 5 button. The game I've been testing with (Adventure AKA AD&D/Cloudy Mountain) does not use the 5 (center) button so I can't really test it, I need to pull my roms for either Major League Baseball or Utopia.

ETA: You'll want to set up quick menu like I did if you need to quit Retroarch/reset your game. You have your choice of triggers, I'm using the select/start buttons.

A picture to celebrate:
Old game screenshot