I would assume that the OP is referring to the guest plus floor spot format or a formal open stage. It wouldn't be less likely in a session as the format is totally different. I don't remember coming across any other format for my first ten years in the folk scene then bar sessions and singarounds started becomming popular.

I have come across people who come in, do a floor spot and leave but that has never been frequent and is nothing new. I can recall it happening at clubs 25 years ago. I haven't observed any sort of trend in that direction. In some urban clubs where people rely on public transport there can be an exodus before the official finish which could be misinterpreted.