Multiplayer games with their servers turned off for good.
Not including games that your favorite streamer stopped playing.
Not including games that don't break their all-time high player counts every second day according to SteamDB.
Not including games that "fell off" according to Twitter/Reddit/Steam.
Not including single-player games with no online multiplayer component.
I might include single-player games featuring Always Online DRM with turned-off servers.