3 days ago they sent me an email informing me about the security issue.

What caused me confusion is that I have deleted my account a couple of years ago, which means there are two possible scenarios here: 1) either they lied to me that they have deleted my account or 2) they have indeed deleted my account BUT they rollback old backups that my account was part of it.

If #2 indeed took place, the real question is this: why have you even retained old backups with accounts that no longer exist?!