Whoever has the shared subscription will be okay. That person can downgrade to an individual plan if nobody else is using it.
If the person who is currently subscribed to and sharing a family level plan downgrades, other members will be without that person's iCloud coverage. If they use more than the free 5GB then they will not be able to add items to iCloud until they upgrade. They need to get their own individual plans before that family level person downgrades.
If you are downgrading as part of now being covered under another person's family plan, read this support article --> Share iCloud+ with your family - 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.
- From:
Downgrade or cancel your iCloud+ plan - Apple Support
Share iCloud+ with your family - Apple Support