THat isn't even a slightly "unbelievable" decision. While it would have been overturned without any trouble with video replay look at where the linesman is. it is literally impossible to be looking at the City player several metres to his left and the middle of the box and call an accurate offside there. It's unfortunate but 99% of linesman would call that offside. That would have been a miracle for a linesman to spot he was onside.
No one for Palace even complained, that is not a single City or Palace player realised it was onside either, 90% of bad calls players involved know, that is how difficult it was to judge/call that onside for anyone on the pitch.