All of the features of Steam that aren't directly tied to having an authenticated purchase of the game on Steam -- so excluding achievements, leaderboards, mult

All of the features of Steam that aren't directly tied to having an authenticated purchase of the game on Steam -- so excluding achievements, leaderboards, multiplayer, DLC -- work via adding non-Steam games to Steam. You can use the game via Proton, you can make use of Steam Input, you can use the in-game overlay, share screenshots, etc. But of course adding a non-Steam version of the game doesn't magically give you a Steam version of the game absent a key-based import (which Valve supports both technically and a matter of policy).