PSA - If your Steam Deck or Steam on Linux has started failing to download or launch Proton games, you may have caught a bad config from Valve's servers.
We believe the problem is fixed now, but you need to delete a cache file locally (`~/.steam/steam/userdata/<steamid>/config/compat.vdf` where <steamid> is your id).
See https://github.com/ValveSoftware/steam-for-linux/issues/10846 for details.