Storage plan canceled, what happens to the data stored?

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Downgrade your iCloud storage - Apple Support

When you downgrade your iCloud storage plan, the storage decrease takes effect after your current monthly or annual subscription ends.*

If you downgrade or cancel your plan and your stored iCloud content exceeds your available storage, some of your data will not be uploaded into iCloud. Learn how to manage your storage by deleting information that you don't need anymore.

If you downgrade to the free iCloud Storage plan, you'll have 5GB of storage.

I made the type above bold for emphasis. If you have more data than you have room for in iCloud, you will lose that data. You need to save it before you downgrade.

Archive or make copies of the information you store in iCloud - Apple Support

After 30 days, the excess data will be purged. When you normally cancel a subscription, you receive a message telling you that. If you’re payment lapses, I assume the same time limit applies before the data gets dumped for good.

The thing about a bulk purge of data, is that what gets left may end up corrupted or incomplete, or at the very least, not retain the 5GB’s worth you’re most concerned about.