What happens to a family plan members data when they leave the iCloud shared family plan?

iCloud keeps all of the information associated with your Apple/iCloud ID for 30 days whenever you have a payment issue or change.

When you joined Family Sharing, your Photo Library did not move or get re-associated with the Apple/iCloud ID that "owns" that data. It is tied to your ID even if you went with the Family Sharing plan. All that does is move the responsibility for paying for the storage from you to the Family Sharing organizer.

All you need to do is leave the Family Sharing plan, and then upgrade your iCloud Storage. Anything that was stored under your Apple/iCloud ID will remain in iCloud for 30 days, so if there is a gap between when you leave (or were removed) from Family Sharing and when you upgrade your iCloud Storage, as long as it doesn't exceed 30 days, you should be good to go.

Cheers,

GB

When you leave Family Sharing, your Apple ID is removed from the family group and you won't have access to any services shared by your family. This might include an Apple Music family membership or a shared iCloud storage plan. You stop sharing locations with your family members and your devices are removed from the family Find My iPhone list. If your family shares iTunes, iBooks, and App Store purchases, you immediately stop sharing your purchases and lose access to the purchases made by your other family members.

Any content that your family shared with you isn't automatically removed from your device. You can purchase it again or remove it to free up space on your device. If you downloaded an app from a family member’s purchase history and made In-App Purchases, you'll need to purchase the app yourself to access your In-App Purchases.

Any DRM-protected music, movies, TV shows, books, or apps that you previously downloaded from other family member's collections are no longer usable, and other family members can't use content downloaded from your collection.

You'll keep any purchases you initiated while part of the family group. However, if the organizer was paying for ongoing subscriptions, you need to start paying for them yourself using a valid payment method on your account. Learn how to add a credit or debit card to your iTunes Store account.

If you share a photo album, calendar, or reminders with family members, you won't share them anymore. If you want to keep using Family Sharing but not share certain features with your family, you can unsubscribe from them instead in the Photos, Calendars, or Reminders apps on your device or on iCloud.com.

You asked: "...What happens to my data? Do I get a chance to buy storage or does it vanish?..." It doesn't vanish, you just won't have access to it any more. However, you won't lose data you brought with you to the plan - if you are sharing your album so they can see it, the album is still yours to keep. On the other hand, if you were viewing their album, it will 'vanish'.

Apple is not going to 'poof' her terabyte of photos. However, they will notify her that she needs to either upgrade her storage plan, move the excess data off the iCloud, or risk it being 'poofed' in the near future.

My impression is that Apple will let this overage stay in place until the next billing cycle.

That is a darn good question, and I cannot find anything that addresses it clearly. It would see that since they are both with different iCloud IDs but are in this shared plan, if one leaves, it would seem the data would be in limbo, since Alice doesn't have sufficient storage for her content. I wonder if she gets a warning if she attempts to leave the shared storage that tells her photos will be deleted. Does she attempt to purchase additional storage under her iCloud ID, and would that be allowed since she is a family plan member?

I believe you may be better off addressing these questions to Apple, since they don't seem to have that currently addressed in a support document.

I described what Alice should do to avoid a potential problem.

Stuart423 is correct that Apple is allowing a grace period to transition without issues, but that is an unstated, voluntary policy. AFAIK, Apple makes no promises of any kind that this policy won’t change, or even that it will be applied to all users.

Alice needs to switch from Family Storage Plan to 2T Individual iCloud Storage plan before leaving Family Sharing.

Change iCloud Storage subscription

https://support.apple.com/en-us/HT207594