It hasn't been removed. Focused track highlighting *is* shown when 'Show strip colors' is off, and is *not* shown when it's on. That's always been the case.
The reason? Lots of different track background colors mean the subtle highlight very often gets lost in all the other colors, to the point where it becomes useless.
For example, you might have a blue track, and a yellow track. Focus the blue track, and the background highlight makes it appear a bit lighter - but it's still significantly darker than the yellow track background! In that situation, the yellow track might appear to be focused at a glance, so what's the point of the highlight? It doesn't work well/consistently when lots of colors are involved.
That said, there are other ways to convey focus of course. Arguably we should investigate alternatives, but for now, that's why we don't highlight focused track backgrounds when 'Show strip colors' is on.