While Stack Exchange can claim that all posts here are automatically CC BY-SA, that would fail when a user deletes their account (or gets banned) as they then become user5876892 or whatever. So instead of the real username, Stack Exchange would show a fake username. As attribution would be required, it would also be impossible for Stack Exchange to conform to this license. Thus, if an account is deleted then all questions, answers and comments by this user need to be deleted also, or they don't conform to the license. Am I right?
So if an account gets deleted, it would become a copyright violation for Stack Exchange and other users to use any contributions by this deleted user. This means that this content can't stay online and will have to be deleted as well. And I mean complete removal, not just hiding them. The alternative could be to keep the user name with all content but just disable the account.
Even though Stack Exchange has an infinite license on using the content, anyone else would have to comply to the CC license and provide attribution, which cannot be done if SE removed the original user name.
Reminder: this question is about the licensing behind Stack Exchange, not about how to delete stuff. The question is if Stack Exchange is allowed to anonymize content when an account gets suspended, as that would invalidate the attribution for the content.