FWIW, Rocket League works just fine on Linux, but you need to set it to the Proton compatibility, not the native Linux client.
How did those issues get fixed? Did they abandon the anti cheat, or does the anti cheat now work under proton?
Do you only play offline or something?
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.