put it in our %Backlog (meaning GitLab, Inc. currently doesn't prioritize it over other items that are scheduled

This issue is going backwards in the process, not forwards. Surely, Gitlab should be focusing on things that have already been allocated milestones, especially ones that have had milestones pushed continuously for 3 years. You can argue that it shouldn't have had a milestone added in the first place, but maybe Gitlab should focus on clearing the "backlog" of issues that have already been issued milestones, and backlog new issues until some of the older issues have been solved.