If the written directive expressly states that clothing styles worn in non-Witnessing activities can be the basis for refusing brothers and sisters in otherwise good standing from engaging in field service when they ARE dressed appropriately, then yes, this is new.
Rather than being new wouldn't this simply be defining what was already at the discretion of the local Elders already? Reproof is reproof, they always could do it for any trivial violation found inside WT literature. The mentioned letter may have some defined terms, but it could have been done in the past anyway, correct?
Clothing was always something to be regulated. A good deal of this was informal, members checking each other through gossip, which eventually could lead to reproof action.