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It's not part of core XenForo, and I don't see any add-ons that would implement it, which isn't surprising considering it's a wholesale change to how the forum software operates. The suggestion has been raised a couple of times on the XenForo discussion forums, and people generally dislike it.

As I'm sure you remember, vBulletin allowed users to customize the number of posts per page...I believe 100 was our max. Unfortunately, XenForo does not offer such an option.
 
I hate infinite scrolling pages. Those are the ones that usually crash my mobile browser.
 
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