I think it was an urban myth. I was on a flight to the US which was held for 40min to allow a large number of people connecting from CPH to get on but other than that I’ve never heard of flights being held for single passengers. If you think about it the complexity of the BA operation would fall apart if they were doing so regularly. A kind dispatcher at ATL once re-opened a flight for my wife and kids to get on but it wasn’t as if they wouldn’t have gone without her if she wasn’t there when they closed the doors.