If I don’t pay my storage

If your card has expired or there is no coverage for the amount, then you will be notified that your Storage will be downgraded to the free plan of 5GB. If you have content that exceeds 5GB your photos will not be deleted, but iCloud will stop updating everything until you either upgrade again or move your photos.

You can read more here:

Downgrade or cancel your iCloud+ plan - Apple Support

Note that depending in how long you leave your account delinquent, yes, content may end up being purged. Apple has the right to delete content when you do not pay for a subscription you chose to commit to. If you cannot afford your subscription, it is always best to make independent copies of important data you wish to keep, and then cancel your subscription yourself. You can always sign up again later.

Legal - iCloud - Apple

“The applicable fee for an upgraded plan will take effect immediately; downgrades to your plan will take effect on the next annual or monthly billing date. YOU ARE RESPONSIBLE FOR THE TIMELY PAYMENT OF ALL FEES AND FOR PROVIDING APPLE WITH VALID CREDIT CARD OR PAYMENT ACCOUNT DETAILS FOR PAYMENT OF ALL FEES. If Apple is unable to successfully charge your credit card or payment account for fees due, Apple reserves the right to revoke or restrict access to your stored content, delete your stored content, or terminate your Account. If you want to designate a different credit card or payment account or if there is a change in your credit card or payment account status, you must change your information online in the Account Information section of iCloud; this may temporarily disrupt your access to the Services while Apple verifies your new payment information. We may contact you via email regarding your account, for reasons including, without limitation, reaching or exceeding your storage limit of your selected plan.”