I may be wrong, but how is GitHub responsive? Here's my dummy way to test responsiveness. In firefox go to responsive mode by Ctrl+Shift+M, now try to play with the width. GitLab behaves responsively by rearranging the elements based on width, GitHub does not (horizontal scroll bar appears).
Interesting, I have the same results in Firefox. When I use "responsive-mode" in Chrome and Safari (and on the iOS simulator) however, they all seem to render GitHub as responsive. I'm not sure that's enough to warrant putting it in features compared to GitHub, though this is up to @sytses