I had a similar concern, but I think I've found a solution. If you right click a game in your Steam library and mouse over to Manage, you have the option to Hide This Game. From there if you click View at the very top of the window the second option down should be Hidden Games which will hide the activity of all the games in that section.I want to support Rem, but I loathe the idea of getting this on Steam, since, well, it broadcasts what you are playing to friends. I originally wanted to buy on ci-en, but now that version is getting censored. Anyone have suggestions?
I've tested it myself kinda and at the very least it keeps your Discord status from changing to "Playing (whatever game)" so I think it should also block out friend activity.