Direct from the horse’s mouth: https://www.ubisoft.com/en-us/help/account/article/closure-of-inactive-ubisoft-accounts/000079595
Edit: The Ubisoft page linked here has almost completely changed in the last day.
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Well, they already fucked with my Rayman Legends save file, even though it was supposed to be on cloud. I have the game on CD but they still require to make an account. Dumb all around.