Not sure the correct place to suggest changes to GitHub website UI/UX.
This suggestion is based around the Branch/Tag switcher modal. Currently the modal has a header, a search bar that is contextual to a tab bar, a tab bar, then under the tab bar is a contextual list of data. From a data hierarchical standpoint this does not make sense. Why would the search bar(which is contextual to the tab) be placed above the tab? It should be placed inside/under the tab bar. The modal can be further simplified by moving the tab bar into the header of the modal itself. I’ve made a mockup to demonstrate my change.