Being able to see a detail view without navigating away from the list view is a better user experience and the more common practice now

I was responsible for this going out. The goal was to provide a more consistent user experience in that what happens when you click an issue would be the same in more places where we use the issue viewer (sub-issues on an issue, our dedicated issues dashboard (https://github.com/issues), GitHub Projects, and others). Like you mentioned, you also wouldn't lose your place when clicking an issue reference when reading a discussion. There were some performance improvements that came with the change too. It was well intentioned, but we hear you, and thanks for the feedback. We missed the mark on this one and it's been rolled back.