Backed-up/stored Data After Device Removal from iCloud?
Is backed-up/stored data lost when I remove a device from iCloud or does the last backup from that device remain in iCloud for future access?
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You can do all of the steps in the following without affecting anything on iCloud. All Backups and synced data will still be there: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support
You can later delete the backup of the old device if you no longer need it.