I don't think that while ever ended! Have you seen Jetbrains' IDEs "new UI" (now the only UI except for a soon to be deprecated plugin)? You can't tell where a tab ends and the next one begins, it's just floating text on a solid background, with slightly more space between names. They also got rid of most labels, so it became a game of guessing what each symbol could possibly mean (another provably awful trend).
And the worst is, they're likely just copying competitors because as a sibling comment days, some people see the old accesible UI and think it looks old fashioned.
I love the new UI. Reduces a lot of visual clutter and lets me focus on the code. I don't understand the problem with telling where a tab ends and the next begins; I don't click on the borders of tabs anyway, just the text. (And there's an icon in front of the tab name.)
And the worst is, they're likely just copying competitors because as a sibling comment days, some people see the old accesible UI and think it looks old fashioned.