The new canvas is the open gl canvas. (developed by the guys at cern.)
Currently in the stable release some features of the default canvas have not been transferred over to the open gl canvas. (There are some guys who say they can not use the open gl canvas because they miss some very important feature. I never used default.)
No new features are added to the default/legacy canvas.
The next kicad release (long overdue) will probably switch to the open gl canvas as default and keep the old canvas as legacy option.
Here the official list of new features only found in the open gl canvas: (Nowhere on this side is it mentioned that these features are only in open gl.)
There are also several discussions here on the forum concerning exactly this topic. But they dent to get derailed. The best one that i could find is this one.
It does focus on the nightly build versions though. So some features discussed there might be missing in the stable releases. (Nightly build is the current development master build every night. So it can be quite unstable.)
I personally only use the open gl canvas. So i can’t give a list of features missing from it. But i can definitely say that the interactive router, differential pair routing and array functions are missing from legacy.
Switching between canvases is done with F9/F11 or in the view menu bar.
(Just switch between them, play around a bit and see what you find fits your workflow better.)