Looks like you can stream it from the Xbox service for free via edge, and you can install a flapak version of edge, if your distro can do flapaks
I just did a quick Google, I haven’t actually tried this so good luck! But it sounds like streaming it is the only way given the form of anti cheat used. Don’t think you can do it natively
Check out this link
https://www.gamespot.com/articles/how-to-play-fortnite-on-steam-deck/1100-6503402/
Looks like you can stream it from the Xbox service for free via edge, and you can install a flapak version of edge, if your distro can do flapaks
I just did a quick Google, I haven’t actually tried this so good luck! But it sounds like streaming it is the only way given the form of anti cheat used. Don’t think you can do it natively