When you did this, was the Trello account used for just your work with that employer? Or for both work and your personal stuff? Or just your personal stuff?
[Edit: I see from your response elsewhere in this discussion that it's the second of the options above. I'll respond further in that subthread.]
but again, to play devil's advocate, Gmail do offer professional account part of the G Suite. Without knowing your work history, it would be difficult to know if the Gmail address is also not a "professional one".
On another hand do you think notification would have solved the issue ? And wouldn't a more malicious employee just delete all the boards if they did not part with their previous company on good term.
Obviously I don't know everything from the story. And my assertions are very far from the truth. I am trying to understand what would motivate such a decision (beside the obvious Atlassian is a heartless money-grabbing company that rot everything it touches)