Why do you think it is unprecedented? They have taken stances on certain fashions before and have also relied on oral instructions that don't always match up with what they print in their publications before so they can avoid being held to account for things.
In AM's rant speech he explained that an elder should (be man enough to) send a person home to get changed if they dressed inappropriately when they turned up for field service. This has now translated into disallowing persons from witnessing work altogether if dressed inappropriately. I don't recall them taking this action in the past, however, as you suggest, they may well have but because it wasn't in any publication it doesn't mean they haven't.
I also thought it was pretty novel that AM's speech, which he specifically said was totally supported by printed publications, has translated into a new policy. The issue of dressing modestly was always there but AM was much more specific and brought up some homophobic ideas which have now been identified to include 'gender blurring characteristics' and brothers who are 'manifesting of effeminate traits, perhaps in ones bearing, body language and manner of speech'.
It was because of a secret BOE's letter having been given to the elders in my congregation a month before my JC that didn't match up with the current procedure's in the congregation that saw me walking out the door without looking back. They weren't even prepared to show me this letter. This secrecy and hidden rules offended me greatly.
Construction sites are a little different to normal - there is some mandated clothing.
It is an extreme example but God's law is supposed to be above man's laws and the same principle applies.
There are also many occupations these days where the required uniform for males and females are the same (although females can usually incorporate skirts instead of pants if they so wish). This includes pilots, nurses, firepersons, fast food employees, etc Again, the org. take no objection to this even though the distinction between male and female clothing is blurred.