(EDIT: just saw the post about the icon being removed )

Here's the direct link to the classic interface:

https://google.com/flights/?f=0

I have this bookmarked and just go directly there Dreading the day when they remove this functionality.

EDIT #2 to add:

This is clunky and barely qualifies as a workaround, but once you have something you want to track, click into the URL field and edit it to remove the "f=0" part (may need to remove a leading/trailing & to make the rest of the URL parameters parse correctly). This will switch back to the "new Coke" interface but preserve all of the selected options/flights and then you can toggle tracking.