I'm not asking about this closing's origins that I understand. Rather, is doing this nowadays sane?
One of my customers, who's not in the British royal family, always closes her letters with "Your obedient and humble servant". I was flabbergasted the first time I saw it, and still literally raise my eyebrows whenever I see it now.
I've been closing replies to her with "Best regards", as I usually do. We're in England.