For those of you complaining, congratulations, you've discovered ~ nostalgia ~
In two weeks you'll inevitably find the old design ugly, and forget GitHub ever looked any other way.
In 5 years, each will get another round of improved designs, and there will be a thread on HN full of people complaining about how the new design sucks and how 5 years ago was "the good old days."
The new GitHub design is objectively better. The new MacOS 11 design is objectively better.
"Low information density" means less clutter. You find the information that matters quicker. Changes to padding/visual separation/sizing/etc. all provide similar context to which information is important, and how items relate. "Flat design" isn't some trend, flat icons are just easier to quickly recognize, and look far more crisp.
In both threads the degree of negativity is disappointing. Can we not have one or two positive comments on how crisp the new commit graph colors look, how nice the transparent pin dragging interface is, or how the action buttons are more prominent? Not to mention the entire code/README page. The flat rounded corner borders are very clean!
If you really don't like the rounded borders, use wget