You guys had me wondering what on earth stays and stay pockets were, until the penny dropped: we call those things collar bones.
We started seeing them in shirts when I was at school, back in the 1960s.
I have never been much of a fan of them, or of button-down collars.
But I am not into reversing collars either.
As for detachable collars, my grandfather caused a scene just before my uncle’s wedding because he wanted to wear a butterfly collar, but he was talked out of it.
When I was a student in a university residence (or dormitory, as they call it in the US), I was required as a freshman to wear butterfly collars for formal dinners. That was the only time in my life that I wore a detachable collar. I think I still have the plastic collar somewhere . . .
Regards,
Mike