When I unlist a topic, it just toggles to ‘unlisted’. There’s no further prompt or interaction.
But it seems that there is “space” for a comment on the action:
And in fact, I can edit-in a comment:
But I have to remember to go back and do that.
IMO it would be nice to be prompted to leave a memo on why the topic is being unlisted so that my colleagues (and future-me) don’t have to re-read the entire thing to puzzle it out.