As a very frequent flier for decades, I’ve seen it all, I’m also a last border onto the plane as I have issues sitting following an operation. Observing what goes on is educational, they take the last arrivals aside and deny them boarding when everyone else is gone, they are waiting for no shows to allocate seats to them, if they are lucky they get on. It’s a very rare occurrence, but you don’t want it to happen to you on the 9 pm flight from Luton to Dublin on the 23 December.Yes, I see what you mean, if the aircraft are switched, there are eight less seats.
Upthread is a link to posts from 2015, including some from me, the past is a far distant country as regards advice, you need to be current with advice, the rules have changed dramatically.