Nobody is kicking him out, the club just told him if he wants to leave, he can. Because the answer is simple, we have already 4 players on that position that are either younger, better or both. Frenkie and Pedri are starters. Gavi is the next in line, Bernal is coming back who is younger and is considered the player with a bigger potential. So Casado would be the 5th choice, which isn't a good situation for anyone.
I think he'll stay for now, but if he doesn't get many minutes this season (which is very likely), he might reconsider next summer. The club knows this as well, so there is no harm done to tell him, if he wants to leave now, we gladly take the money for the ffp.