iCloud data retention for accounts over 5GB?

If you were paying for iCloud+ and stopped or cancelled the subscription, Apple offers no specific time frame for the existence of the data. Storage should be restored as soon as possible to avoid data loss, or data should be backed up prior to stopping the subscription.

click here ➜ Downgrade or cancel your iCloud+ plan - Apple Support

I believe the data loss is immediate and unrecoverable if you exceed your iCloud storage.

From the article Phil0124 provided:

“If your iCloud storage exceeds your new storage plan capacity, iCloud won't sync or update your information, and iCloud backups won't complete.”