Go to OSX Daily to read How to Set Messages to Automatically Delete on iPhone or iPad
Do you want all of your Messages to automatically delete themselves after a specified time period from your iPhone or iPad? You can configure a Messages History setting that will allow for the automatic removal of message conversations after a chosen period of time.
This auto-delete Messages setting is perhaps most useful as a storage saving mechanism and should not really be considered a privacy setting for a variety of reasons, and it has some obvious downsides too, mainly that you could potentially forever lose some great memories, conversations, chat…
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