Banning rehsade in this case would make a lot of people stop playing the game, resulting in potentially me, as a developer, losing a lot of money. For example, if my game does not have colourblind filters, why would I ban people for using reshade, even if it allows people without colourblindness make the game brighter? Both use cases are okay because they are actually very similar - in both cases the players want to be able to see what's happening in the game.
In many games, using 3rd party software is allowed if it helps solve core design issues, accessibility issues or provides cool functionality that the base game does not have.
What's considered cheating in games is defined solely by the developers, as they are the ones who set rules for the community.
