Apparently calling someone by their last name without any title (Mr, Mrs, Dr, Professor, etc) is considered rude. As someone who mainly goes by their last name nowadays without any sort of title (especially online), I find this weird.
If you don't have permission to do so, or are not close or not very familiar with the person, then Yes they might consider it rude and disrespectful. Better to call them by their first #name than call them by just their #surname.
Generalistic and moderated instance.
Everyone is welcome as long as you follow our code of conduct!