That surprises me. I feel I talk about this very subject every fortnight or so, and while I appreciate there are quite a lot of posts by me and others in this forum, nevertheless it is very much a longstanding fact, and I think a well known fact, and printed on some (but admittedly not all) boarding passes. If they are trying for an on-time departure and getting towards 20 minutes with only a few passengers missing, then yes I'm afraid they will often start shouting something "last passengers for BAnnn, we are about to close / offload". That may come across as a threat, which actually it is, but one person's rudeness can be someone else's call for action. In somewhere like B, people do get confused by gate numbers (self included on a bad day) and so the shouting is potentially helpful, particularly for those just emerging from the transit lifts / escalators. If it was T-19 then if they had offloaded you then in the absence of other factors you would have merited that outcome. Normally, however BA will rebook on the next service in that scenario.