[2021] Am I just slow (this is probably the answer) or is this jewel socket summary including exact values for variable mods new?

Jewels have always done this. Jewels don't give you stats directly, they apply stats to the passive which is the socket. You get those stats in the same way you get the stats of any passive - the jewel socket is allocated, so the stats of the socket are granted to you. So you're seeing stats in the passive popup for the socket passive, just like any other passive you hover.

It's been there a long ass time already. But it's also pretty hard to really notice, because you have to hover over the socket without hovering over what's socketed inside.

Anytime you're hovering over the jewel, you're also hovering over the socket passive, so can see the popups for both. Hovering over the edge of the socket only (and thus not the jewel) is only needed if you want to see the passive popup for the socket but not the item popup of the jewel for some reason.