MrPedantic“
Hello Anon
"Yours truly" is mostly used in American English, so I'm not too sure. Maybe a passing AmE will let us know.
MrP
”
Passing American here.
Having spent most of my day today drafting business letters (dunning letters, actually), I can honestly say I have never closed a business letter with anything other than "Sincerely," In no way is this considered casual or informal in business use in the U.S.
I don't know that I have ever used "Yours truly," but I suppose I would use it in personal correspondence to someone I don't know well. For example, if a friend of a friend had hosted me on a visit, my "bread and butter note" might be closed that way. Since we are a shockingly casual race, we tend to end letters with things like "Thanks again," or "Looking forward to seeing you soon," and then signing our names.
Sometimes as I find myself typing "Sincerely" I have this passing fanciful notion that somewhere in a vault are all the letter that were written insincerely, but were not allowed to be sent out.