Permanently deleting a device from iCloud backups
I want to delete an iPad from my phone because the data shares and it won’t let me backup but I don’t know what will happen if I do it
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If you turn off iCloud Backup for a device, any backups stored in iCloud are kept for 180 days before being deleted. If you delete the iCloud backup for your iPhone or iPad, iCloud stops automatically backing up the device.
See View and manage iCloud device backups - Apple Support
Content synced to iCloud outside of device backups will continue to be synced between devices. For example, if you have Messages on iCloud turned on, both devices may sync Messages. See What does iCloud back up? - Apple Support and Manage your iCloud storage - Apple Support