We certainly understand that you are unable to login to GTA online through Steam Deck after the update.
We would like to inform you that the primary goal of incorporating BattlEye into GTA Online is to enhance the overall gaming experience by actively scanning for cheats and exploits, and preventing players with cheats and mods from entering online. The Steam Deck players will not be able to join GTA Online but should be able to launch single-player without any issues. We appreciate your understanding in this matter.
If you have any further questions or concerns, please feel free to let us know. We are here to assist you.
Rockstar replied to someone’s support request
Source https://www.reddit.com/r/linux_gaming/comments/1fizrqb/rockstar_official_replied_that_they_wont_support/