Destiny 2 is not supported for play on the Steam Deck or on any system utilizing Steam Play’s Proton unless Windows is installed and running. Players who attempt to launch Destiny 2 on the Steam Deck through SteamOS or Proton will be unable to enter the game and will be returned to their game library after a short time. Players who are not accessing Destiny 2 through Windows and attempt to bypass the SteamOS/Proton incompatibility will be met with a game ban.
https://news.itsfoss.com/destiny-2-linux-bungie/
Bungie being a bitch as usual