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That means all the messages and attachments which are older than 30 days or more will be deleted automatically. iPhone does this for you automatically. This is more like an automatic cleaning process. If you manually delete any messages regardless of the age, they will be deleted permanently and can't be recovered.
If you have any questions or would like any additional information, please don't hesitate to ask
Even the messages that are deleted after 30 days will be permanently deleted and you wouldn't be able to see them on the device.