What happens to my data if I cancel my iCloud Storage subscription?
According to this Apple iCloud Support page:

If you downgrade your storage plan and your content exceeds the storage you have available, new photos and videos won't upload to iCloud Photo Library and your devices will stop backing up to iCloud.
iCloud Drive and iCloud-enabled apps won’t update across your devices, and you won't be able to send or receive messages with your iCloud email address.
To use iCloud after exceeding your available storage, remove backups, photos, documents, or email messages you don’t need. You can access photos, videos, and documents for 30 days after you delete or disable iCloud Photo Library and iCloud Drive.