And it is on by default, though you can return to the new method if you want to (but why would you). She is also reporting that the tab bar will make a return on iPad OS 15 in a future beta.
I had posted about this and was using it in Safari Technology Preview in Big Sur. A new version hasn’t been released, but I will take a look at it once it comes out, to see if it is improved. Now if only Apple would add the option of tree style tabs, ha one can only dream.