Well, it seems that it both has and it hasn't.
Yes, I can now select a bunch of songs, right-click, select "Get Info" and paste artwork into the small box presented there. But when I do, the artwork doesn't show up. Not immediately.
In 10.3 when I would paste artwork in that fashion the art shows up (albeit in very small thumbnail form) right away. The first time I tried this after upgrading, I put in some artwork and it was just blank. I cancelled the screen and tried again. I tried different art formats and various other techniques. When I finally quit cancelling out of the "Get Info" screen and clicked the OK button instead, then the art showed up.
I wonder what other code was being fixed when this broke, because it seems like a very odd thing to go wrong. And to my humble opinion, it's only partially fixed at this point as it does not restore the full functionality that was present in earlier versions.