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That's what I'm wondering. I know for calls this kind of thing was implemented as part of CallKit, but I'm not quite sure if that includes anything for messaging, and in particular iMessage which is controlled by Apple directly essentially.

Right. Check out the app (RoboKiller). Perhaps you can determine how they do it. I’m afraid their explanation is vague.
 
You can turn on Restrictions on iPhone or iPad by going to Settings --> General --> Restrictions --> Websites --> Limit Adult Content
So have to either lose ALL porn access or risk a phone-glitching bug spread by text?

*deletes Messages app entirely*
 
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