That is not an unwritten rule.

Just like it's not an unwritten rule that you can't celebrate a goal against your former club. Just like it's not an unwritten goal that you have to stand behind the service academies during their alma mater.

Some individual teams, clubs, players and managers show respect to their opponents in different ways.

The ref controls the game. If there is a dangerous situation he can blow the whistle to stop play. Otherwise, some folks kick the ball out. But players would be fools to expect as much. That being said, Leeds went afoul by acting like they were going to play it out and then going in for the goal after the players stopped.