Since Wine and Proton both don't offer a "native" interface for games running under them to connect to the Discord RPC listener on Linux, here's a short guide on how to make RPC work.

I used this method on Manjaro 20.2.1 'Nibia,' though it should work the same on all other distros too.

• An internet connection
• Wine/Proton already set up with winetricks/protontricks installed
• This tarball by mellowagain
• A Terminal

• Download the tarball and extract it to a folder. You can extract it anywhere, but I'll be using /opt/discord-rpc
• Make sure you have the bin64 and the bin32 folders in /opt/discord-rpc
• Open a terminal and append the locations of the bin64 and bin32 folders to the WINEDLLPATH environment variable using the following command export WINEDLLPATH=$WINEDLLPATH:/opt/discord-rpc/bin64:/opt/discord-rpc/bin32
• Run winetricks/protontricks, select your wine prefix (default prefix if using protontricks), and run winecfg
• Head to the libraries tab, and add a new override for library by typing in discord-rpc and clicking add. Hit Apply, and then OK
• Run your game the way you usually would, and RPC should be working now

Note: This has to be done for each game individually. There might be a way to set it up for all wineprefixes at once, but I'm not aware of how to do that.