Because you say "building=fuel_station" is not correct, how would you tag a fuel station building, that is not longer in service, so "amenity=fuel" can't be used to mark it.

I found a video of some guy who converted a old fuel station into a house, where he can live. 
https://youtu.be/KSfv9agNDkA

To mark it is used for living, it gets "building:use=residential". But how should i now tag that the building is still a fuel station?

Maybe "building=fuel_station" should be documented in the wiki instead of deleting all instances of it.

...and did you check if all the fuel stations you changed are still in service?, Maybe some mapper thought that there were some uses of the value, and add it to a fuel station that is not longer used as one, because there are a lot of tags that aren't documented at this time, but still in use (although often used mixed and on different things, that shouldn't be mixed in one tag).