How do I create a Rule "When role is removed, send email to the user"?
It has to be triggered when a particular role ("subscriber") has been removed. So we can use 2 conditions User has role(s), to check if he had the role before the edit, and that he doesn't have it after the edit :

1st condition : selector "account_unchanged", check the role "subscriber" (here test)

2nd condition : selector "account", check the same role, and negate.