Hmmm how? How can they decrypt your already end-to-end encrypted and uploaded data without you entering the passphrase to do so? I can understand them removing the data from iCloud completely, or asking you to send the keys to Apple, but I don't understand how they can disable the feature for already uploaded data.
When a user turns off ADP in settings, their device uploads the encryption keys to Apple servers.
I can't imagine they'll cancel your iCloud+ subscription. ADP is not a feature of iCloud+ and iCloud+ has features beyond extra storage space. Nor can I imagine they'll delete your data preemptively as long as there's space to store it.
Hopefully they'll provide instructions on how to manually delete your iCloud data in case you don't want to use it any longer (I think you just turn off iCloud on all your devices).
These companies have to comply with so many laws and want cozy relationships with governments, so they play both sides. It likely does things differently, but if the keys are not secure, then its not secured