A number of people have reported this happened to them in the past. I think it's due to Jagex determining that the activities on your account were too severe to ever warrant an unban and so they deleted the account altogether.
But I have been seeing people state that their account just no longer exists, which is obviously totally different than being banned, as with being banned you actually have something to log in to which will state that you're banned.
It's not banned, it's deleted. There's a noticeable difference.