If you consider the backlog as something like the "inbox" for a project (kanban style), it might very well be useful to see how much work the project got stuffed with from outside, so this should at least be configurable.

This would prevent you from moving anything back to the backlog. It doesn't seem completely unreasonable, but if this was the default behavior I'd worry it would be confusing.

I assumed backlog was always there, so don't think it's too confusing.

Oh, I mean it would be confusing if backlog vanished when emptied. You don't think that would be confusing?

Specifically, I'd expect users to ask questions like "Project X isn't showing a backlog anymore, where did it go?"

Oh, I read this completely backwards. I agree with @epriestley, expectation is every panel always exists so you can drag to it. Plus, we need a catchall for un-sorted tasks.

Offhand @tcook, does this behaviour exist in competing products (I am not super familiar with all competitors).

No, since there's no way for a person to drop cards into a workboard without also categorizing them into a list (in competing projects). The "Backlog" column has to exist in Maniphest because anybody can add cards to projects, and they have to go somewhere on the workboard.

I'm ok if the backlog list is always there, but when it's empty it just uses up valuable horizontal space. Maybe with the implementation of T5671 it could just be dragged over to the far right and be largely unseen in day to day work.

I do plan to let you move it around, we just use the position as a magic flag right now and need to fix that.