Considering that it would be an invasion of privacy and confidentiality for Atlassian to access the content of the board to assess which one is corporate and which one is personal, Atlassian to the safer approach to satisfy a paying client.

Consider that as a free user, with no advertising to monetize you, one could guess that Trello used you for advetising (Unless you are a paying user for your personal account, that could change the story).

Of course I am not big fan of the approach, because the user probably linked personnal and work account for convenience, and that trello probably didn't make it easy to make the switch between work and personal.

On the other hand, how do you prove that an email address is a personal one ?

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)