Shadows are not a problem in photography, in fact light tells a story, builds atmosphere, drama ....
The obsession with flat shadow-less lighting was probably for the color printing process back in the day.
But now we can have dramatic expressionistic lighting and photography, yes it allowed, the "rules" Markham talks about are very rarely enforced if ever, only result in a minor fine.
in fact I can find no reference to them in Langford's "Basic Photography" or his "Advanced Photography" . I hear they are mentioned in a rare and difficult to obtain e-book.