Unless something has changed very recently, Destiny 2 does not work under Proton because the Anti-cheat isn’t compatible. Windows is an option, but I’d really recommend a dedicated Windows handheld for that instead of the Deck.
I haven’t tried Windows on the Deck itself, but based on 6800u performance, you’re looking at maybe getting a solid 30fps on lowest settings. Destiny 2 is not a very well optimized game atm, and even the Ally at 20w can have trouble holding good performance in Neomuna or the Tower.