The Steam Deck is software-freedom friendly

How good is platform support - can I install e.g. Diablo 3 and have the thumbsticks control the character and not a virtual mouse? Or would I need to remap the input myself?

It varies by game - if the devs created a default control scheme, the Deck will load that by default and you're good to go. (The green check mark means that Valve verified this behavior, as well as other things.) And if not, the Deck UI makes it pretty easy to create your own mappings and share them, or download "community layouts".

You don't need a Deck to try it - you can run Steam in "big picture mode" on any computer, with a controller, and get the same UI which the Deck uses.

Steam's Input system is great, and many people will launch non-Steam games through Steam specifically for the controller mapping. Any game with controller support (console) works flawlessly with the Deck, or any PC using a controller. I don't think I've come across a game that doesn't work out of the box.

I was going to check that and I realized i can’t find diablo 3 on steam. I know I could install it myself through some other online store. Still, I wish there were cartridges that “just worked”; consoles (including today’s Nintendo) got that part right IMO.