Back in the day Ashley Cole was given dogs’ abuse for going from Arsenal to Chelsea, but there just seems to be no proper animosity now from fans towards former players. Cech, David Luiz, Giroud and now Havertz have all gone to the other side, but no one really seems to care and all those listed seem to still be well respected by their former club (apart from Havertz, who no one seems to care about either way).
Why has this rivalry died off? Arsenal and Tottenham players would never transfer to each other, nor Man United players to city or liverpool etc.
It’s a case by case basis, depends on the player and the circumstances at the time but what tends to be mutual and consistent is that we wish whoever makes the switch nothing but the worst (at club level).