I find that this is more intuative to use than a hard on/off only.
Ideally this should be an option. I get that 'modern UX' is all about more screen space and whatnot, but I'm not a fan of that trend personally. It seems that all that's been gained from that change is more empty space at the side of pages. Personally, I'm the type that would rather have more information and convenience on the page than extra space.
Would it be that hard to add an option to enable the side bar if people want it? That's what I like about GitLab, that it's very configurable.
We are definitely focused on usability, and navigation in general. We don't claim to know all the right answers all the time. So we are iterating with each release, and working hard doing user research as well to inform our decisions. You can see some of our recent work regarding nav here: https://gitlab.com/gitlab-org/ux-research/issues/3. Thanks.
There's lots of discussion there. We went with a design that leans on getting out of your way and recovers some screen real-estate. We do recognize the one additional click required though. And we are actively listening to feedback and will continue to iterate.
We're working hard to improve usability and navigation in general. In addition to this sidebar change, we've made many nav updates: https://gitlab.com/gitlab-org/gitlab-ce/issues/26348.