I have this problem too. I drag links to a tab all day long (or at least I used to until the 15.0 update). It's great when I do a search, then I start dragging search results to a tab until I find a relevant page.
Now, when I drag a link to a tab, nothing happens when I hover the dragged link over the tab. When I drop the link, the link races back to its position on the page I dragged it from and nothing open is the tab I tried to drop it on.
Now, instead of a one handed drag operation, I have to command click (requires both hands) to open the link in a new tab, then delete the old irrelevant tab.
The link to the "Use tabs for webpages in Safari on Mac" doesn't mention dragging and dropping links on a previously opened tab.
I hope this will work again soon. It's a very big part of how I use Safari. Other browsers still allow the functionality.