iCloud data after subscription cancellation

If you cancel or downgrade, your data isn't deleted right away, but you drop back to 5 GB. If you're over that, iCloud stops backing up and syncing new data, and eventually new files/photos can't upload. Before cancelling, download your photos and files (privacy.apple.com > Request a copy of your data) or copy them to your computer. Check Settings > [your name] > iCloud > Manage Storage to see what's using space.

If you downgrade, make sure you read the section "What happens when you downgrade or cancel iCloud+" in this support article https://support.apple.com/108318

What happens when you decrease your storage plan

● If your iCloud storage exceeds your new storage plan capacity, iCloud won't sync or update your information, and iCloud backups won't complete. iCloud starts syncing again and iCloud backups resume when you increase your storage capacity or delete information to make more space.

● If you downgrade to the free iCloud plan, you will no longer have access to iCloud+ features like Hide My Email, Private Relay, and HomeKit Secure Video support.

● If you downgrade or cancel your iCloud+ plan, the change takes effect after your current subscription billing period ends.

● If you pay for both Apple One and an iCloud+ plan, you can downgrade or cancel the iCloud+ plan and just use the iCloud+ plan included in Apple One. After you select Change Storage Plan, there might be additional steps before you can downgrade or cancel your storage. 

● If you leave a family that shares an iCloud+ plan and you're using more than 5GB of storage, you can buy your own plan [ https://support.apple.com/kb/HT201318 ] to continue using iCloud storage.

● If you choose not to buy your own plan and the content that you’re storing in iCloud exceeds your available storage, new photos and videos won’t upload to iCloud Photos, files will no longer upload in iCloud Drive, and your iOS and iPadOS devices will stop backing up.

Apple does not say that you immediately lose any data but it does say: https://www.apple.com/legal/internet-services/icloud/en/terms.html#:~:text=%28g%29%20failure%20to%20pay

VII. B.

"Apple may at any time, under certain circumstances and without prior notice, immediately terminate or suspend all or a portion of your Account and/or access to the Service. Cause for such termination shall include […] (g) failure to pay any fees owed by you in relation to the Service, provided that in the case of non-material breach, Apple will be permitted to terminate only after giving you 30 days’ notice and only if you have not cured the breach within such 30-day period."

This could be interpreted as meaning that if after 30 days you are not paying for the larger amount of storage that they could terminate that part of the service (i.e. delete the files).

"Icloud subscription: what happens to my icloud data if i cancel the subscription"

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Export for Safe Keeping:

It isn't deleted right in an instant --you have until the monthly subscription ends. So, what you should do is export all of iCloud drive to an external hard drive. Then, verify you can access the data. Once verified, you can trash what is in there, or just keeps it while it lasts. Being locally shored, it is there for safe keeping. Use my User Tip: Getting a Quick and Direct "iCloud Drive-to-External Hard Drive" Download - User Tip