Account deleted due to some unknown violation of Apples T&C.

• I have never found where Apple cites a specific violation for immediate termination, beyond the statement of a violation of the T&C. I have seen where Apple notified a user who was sending a large number of Messages that others considered as Spam and was warned not to continue, but apparently from you post a severe violation is at issue here. It is also the case for most other companies as well, and seen the same reports by users of TikTok, Microsoft, and even banks that will close an account without notice and without explanation. It some cases they are prevented from telling you by law, and other times it just may be their policy.
• Your data should always be backed up. Even important data synced to iCloud should be backed up to a computer or external storage device.
• It is unusual that a user would have no idea what they did. Not sure if reviewing the T&C would help.

Legal - iCloud - Apple

• The relevant portion of the T&C on removal of the account is here. There is no appeal process and the only one I would expect they would tell you about is if they had received a cancel request from you, which is obviously not the case.

Apple may at any time, under certain circumstances and without prior notice, immediately terminate or suspend all or a portion of your Account and/or access to the Service. Cause for such termination shall include: (a) violations of this Agreement or any other policies or guidelines that are referenced herein and/or posted on the Service; (b) a request by you to cancel or terminate your Account; (c) a request and/or order from law enforcement, a judicial body, or other government agency; (d) where provision of the Service to you is or may become unlawful; (e) unexpected technical or security issues or problems; (f) your participation in fraudulent or illegal activities;