As a product manager, I would like to have an overview about the completion status of epics in the backlog. The "Epics" panel in the classic backlog allowed to see progress information such as the count of issues (helpful for teams in Kanban or with noEstimate Scrum), count of completed and unestimated tickets and the total story point estimate. Also, there was a progress bar showing how many of the story points for tickets assigned to this epic were done.
This was extremely helpful when prioritizing features across differen epics for the next sprint, as it allows to quickly grasp which of the running epics (we typically work on multiple epics within a sprint) can be finished.
Finally, the ability to assign tickets to epics via drag and drop on the epic panel was one of the biggest time savers which is no longer available.
I am aware about being able to filter by epics on the "next gen" backlogs and also to assign via the "add to epic" function - but for me the UX here got clearly worse and I have to spend more time doing the same planning than before.