Actually found some guidance in the privacy policy:
https://www.apple.com/legal/privacy/law-enforcement-guidelines-us.pdf
section III, part G, subpart iii:
iCloud stores content for the services that the subscriber has elected to maintain in the account while the subscriber’s account remains active. Apple does not retain deleted content once it is cleared from Apple’s servers.
It’s not clear, however, what constitutes something being “cleared from Apple servers”.
Does that happen when a customer deletes data from their iCloud, or when Apple deletes it? If Apple retains even deleted data for a period of time, then it would mean that your content isn’t truly deleted right away.