What? They use Easy Anti Cheat, which is one of those nasty kernel level anti cheats. Even ignoring my personal experience with the issue, a kernel level anti cheat built for windows seems quite unlikely to not have any issues whatsoever on Linux...
Rocket League literally has a platinum status on ProtonDB[0]. It works perfectly on Steam using Proton if you have it there, or Heroic[1] if you have it through EGL.
Rocket League has literally never been unable to run on GNU/Linux. The full game. Multiplayer, Steam workshop, everything.
I suspect you may be thinking of a different game honestly. Maybe League of Legends? I think they switched to an invasive anti cheat a few years ago that caused it to no longer work on GNU/Linux.
How did those issues get fixed? Did they abandon the anti cheat, or does the anti cheat now work under proton?