Celeste's implementation of difficulty is not perfect, imo.
Having a whole campaign 'tarred' as assist mode only is a real shame. They could have allowed individual collections or progressions as 'assist mode enabled', and allowed the player to challenge themselves to remove the marker. This could work in Wargroove by allowing players to repeat the levels again on normal difficulty. Into the Breach marks its challenge trophies in different colours based on the difficulty chosen, so players can return the game on a later difficulty, or recognise that one particular trophy had to be done that way.