and yeah with Proton version 7.0 the nameplates are back :

Hello @tatsujb, we're using one issue report per unofficially supported game title, so I've gone ahead and transferred this issue report to https://github.com/ValveSoftware/Proton/issues/3521#issuecomment-1186181813.
Compatibility Report
System Information
I confirm:
(I'm creating a new thread since rocket league worked fine before, this is a new issue that suddenly surfaced. I tried reporting it to Psyonics, I also made a reddit thread but it's when another said the issue went away for him when he downgraded proton version that I realized this might be due to a proton update : https://www.reddit.com/r/RocketLeague/comments/vyulfb/comment/igcagvi/?context=3 )
steam-252950.log
Symptoms
player Nameplates don't appear since the 12th of July 2022

as you can see neither my teamates nor myself have a nameplate and that makes players hard to see.