Retrospectives are a common place for this, and I think a fairly healthy one. They're limited in a way that meetings aren't, and they're already structured to take informal comments and process them into useful actions. I've seen too many meetings that amounted to "complaining about external blockers", and unlike retros there's almost never a useful followup.

I've never seen "managers aren't permitted", but I suppose it depends on the level of manager - as long as everyone present got their hands relatively dirty on the project, things seem to go well.