When Valve introduced its Steam Machine cube gaming console/PC, the gaming community began questioning the hardware choices and Valve's performance claims. However, a Valve engineer stated that the Steam Machine is more powerful than 70% of gaming PCs on the market, based on Steam Survey data. It fe...
https://gpuopen.com/fidelityfx-super-resolution-4/
It’s possible they add compatibility at a later time (with reduced performance and/or quality due to lack of hardware acceleration), but they haven’t announced anything like that currently
It’s not a hard requirement. Gamers have used the SDK to get FSR4 working on Steam Deck. Here’s a video of it in action.
https://www.youtube.com/watch?v=95N6-2U5YQo
TL;DW – It’s good to have as an option but not necessarily better than FSR3 on Deck due to tradeoffs. Depends on the game.
Yeah you can hack it in but that’s not official, which results in what you see: performance is not so great. That video only shows a small performance loss because FSR4 is only outputting to 720p, but making FSR4 output to 4k (like a normal TV) is a much bigger demand. But maybe some specific optimizations can be made.