It would need a lot of internal changes to XenForo because the push subscriptions are stored by user ID (which guests don’t have).
Besides the fact it would take a complete rework of how XenForo’s push notification system works, I doubt we would ever see it. At the end of the day what purpose would it really serve? Just a way to spam people that didn’t like the site enough to register for an account about things that isn’t relevant to them (blasting out a push notification to 40k unregistered users about a random thread is literally the definition of spam, imo). So if it was up to me, a feature like that wouldn’t make its way to XenForo core. Maybe useful for some vertical market that I’m not aware of, but I can’t personally think of any site that I didn’t want an account at, but I did want them to send me push notifications about stuff not relevant to me.