This is indeed a Default feature - see the first photo below from the Default xF script in Site Admin -
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Under 'Preferences' Profile Hover is already there - do you see this? If you click on 'Profile Hover' under 'Preferences' you get to the 2nd photo below:
View attachment 46127
And, Peter subsequently comments "I have not been able to figure out where the Profile Hover field comes from. It's not added by any of the add-ons, because I disabled all of them, and the field still showed. I actually do not think it's a default feature, as I do not see it on my own sites".
So, please comment on this,either Brogan or any one else from the official xF team.