If you turn off Messages in iCloud on an iPhone, iPad, or iPod touch, your message history will be included in iCloud Backup. If you set up an Apple Watch for a family member, Messages in iCloud is the only way to back up messages.
Does turning off messages in iCloud delete messages?
What does disable and download messages mean? Disable and delete messages will save all your messages saved on iCloud across all your devices.
What does turning on iCloud for messages do?
You can sync your text messages across Apple devices by enabling the iCloud messages sync feature. Synced messages will all be viewable in the Messages app, whether you’re using an iPhone, iPad, iPod Touch, or Mac. You can only sync your text messages to iCloud on Apple devices.