I agree that it has less relevance, but it is still relevant for anyone working on projects with more than one track. It appears to be largely irrelevant because for mouse users the focussed track is implicit.
if you click on a solo button or a mute button, you are changing the solo / mute status of “one” track, but it is irrelevant whether that track is selected or not.
If you press the context menu key, the context menu for one track opens, and it is irrelevant whether that track is selected or not.
If you press “Enter”, the selection of one track will toggle on/off.
What do we call the “one” track that is being acted on? You could call it the “active” track, or the “current” track. The Audacity documentation calls it the “focused” track.