They really didn't actively think it was a good idea. They changed the discovery tab to be a web page, which as the github repo linked by the OP will show you, is now handled in the same repo (by the same people) as the…
There are sidebar tab WebExtensions, including one rudimentary one which supports tree indentation. I even contributed to one this morning that is quite attractive, called Tab Center Redux.
They really didn't actively think it was a good idea. They changed the discovery tab to be a web page, which as the github repo linked by the OP will show you, is now handled in the same repo (by the same people) as the…
There are sidebar tab WebExtensions, including one rudimentary one which supports tree indentation. I even contributed to one this morning that is quite attractive, called Tab Center Redux.