Purchasing iCloud Storage
What happens when I stop paying my monthly bill? Does that storage just get deleted so all of my pictures? How will I not eventually pay off my purchase so I don’t keep paying?
iPhone 8, iOS 13
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iCloud storage is billed monthly for as long as you continue to want to use it. You don't eventually pay it off. It is similar to renting physical storage space or renting an apartment; you continue to pay as long as you want to use the space.
Following is Apple's statement about what happens when you cancel. From Downgrade your iCloud storage - Apple Support
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.