Anything using a compatibility layer (e.g. Proton) through Steam is going to have an entry in the ‘compatdata’ folder in your Steam library. Inside that, there’s an entire windows filesystem folder structure, so finding the actual data is a two part process:

• Find your compatdata folder in your Steam library; usually you can do this by rightclicking a game in Steam -> Browse Local Files -> go up 2 folder levels (to steamapps) - should be a compatdata folder in there. Open that, find the folder whose name matches the app ID, and you’re in business.
• Navigate the fake Windows folder structure to wherever the save data would be stored in Windows. [user] is always ‘steamuser’.

It looks like a really obtuse file path because it’s essentially two filepaths in one, but it’s not as bad as it looks to actually navigate.