If I downgrade my iCloud storage, do I loose my iCloud data?

You know, Apple doesn't actually say you lose it. They just say it won't update, nor will other items sync.

If you downgrade, make sure you read the section "What happens when you downgrade or cancel iCloud+" in this support article Downgrade or cancel your iCloud+ plan - Apple Support

What happens when you decrease your storage plan

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

● If your iCloud storage exceeds your new storage plan capacity, iCloud won't sync or update your information. iCloud updates again 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 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 [ Upgrade to iCloud+ - Apple Support ] 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.

However, there may be things in the full terms fine print that might result in data being deleted since you are no longer paying for a service that is being used: Legal - iCloud Terms & Conditions - Apple

Limnos wrote:

You know, Apple doesn't actually say you lose it.

I agree, not in any easily accessible public webpage no, but elsewhere they do, although obliquely, and it isn't easily found.

In section III, part L, subsection I c of this privacy policy https://www.apple.com/legal/privacy/law-enforcement-guidelines-us.pdf referring to iCloud they say:

iCloud stores content for the services that the customer has elected to maintain in the account while the customer’s account remains active. Apple does not retain deleted content once it is cleared from Apple’s servers.

What it doesn't say is how often the servers are "cleared", or the definition of "active" but it does mean that there is some sort of regular sweep of accounts which have abandoned their storage space. Time and again I have seen Apple Community Specialists warn in posts like this one that data may be lost.

Therefore I think it is unsafe to assume data will remain for any length of time on an iCloud+ space for which the subscription has been cancelled or downgraded.

Yes. If you downgrade to a data allowance below what your data consumes you will lose it so make sure you download it and preserve it somewhere.

Apple has some notes at Downgrade or cancel your iCloud+ plan

I think its 180 days that they remain on apple servers. At least thats what apple has told me.

Interesting. I read the full paragraph but it seems to be about if a customer chooses to delete their data then the data are removed and Apple cannot recover them. That's If the customer deletes their own data, not Apple deleting it.