There are some strings attached to this one in that getting it requires Amazon Prime and is probably limited to the USA, but ENDER LILIES: Quietus of the Knights is a spectacular game with a platinum compatibility rating on ProtonDB, currently available free via Prime Gaming.
The second string is that it's activated via Epic Game Store, so you'd need to sign up there and then use Lutris or Heroic to get your hands on it. (Or run the EGS client itself via Proton, but why?)