this is a guide to get Fallout 2 working well on the Steam Deck with "Scaling X2" option to make the text and graphics more readable. if you don't want Scaling X2 then 1280x800 works perfectly fine but still skip to third to last paragraph (last image) to get rid of slowdown.
no launch options, no patching or messing with the game files, just using the default Steam install. this was done on an LCD Steam Deck but it should work for OLED too. this may work with mods or extra unofficial patches but i don't know and probably can't help you there.
install Fallout 2, Steam will default to proton 9.0-4 compatibility. if it doesn't at any point in the future, select that under Compatibilty in fallout 2's Steam game settings.
in Fallout 2's Steam game settings (gear icon to the right of Play button on Fallout 2 game page, then "Properties...") select 1600x1000 in Game Resolution dropdown menu and enable the option right under it that says "Set resolution for internal and external display". this is to get the Scaling X2 option in a 16:10 resolution since the game won't let you do that at the Steam Deck default 1280x800.
launch Fallout 2, click "Play" on the Fallout 2 launcher. on main menu go to "Options" then "Screen Settings". manually type in 1600 width and 1000 height resolution using the on-screen keyboard, you have to press enter after typing in each number for it to stick. enable "Scaling X2" option on the top of the menu.
click "Done", the game WILL go to a black screen. just exit game from Steam button menu and relaunch.
if the game pops up something on relaunch like "Cannot draw resolution 1600x1000" then you didn't enable the "Set resolution for internal and external display" option in the fourth paragraph (third image) of this page. do that and it should launch.
once back in Fallout 2, press QAM button ("..." button), go to the lightning bolt icon tab (Performance), enable "Use per-game profile", then enable "Disable Frame Limit" option. this removes the slowdown that plagues the game without it.
also, i highly recommend disabling Steam Cloud saves for Fallout 2 in the Steam game settings as seen in the bottom of image 3. they don't transfer between machines correctly and won't be usable anyway.
if you want my Steam Deck controller config then seek out "Leon's Fallout 2 Deck Config" in community configurations. it has named inputs and uses long-presses for button double functionalities.