New? This has always been a rule since they started coming out with those types of jeans now well over 30 years ago. i definitely remember we had COs and circuit conventions where it had been mentioned in the late 90s and early 2000s.
The way it was always explained was that we should always be ready to represent Jehoober and never wear anything reflecting rebellious attitudes or rock and roll or heavy metal or emphasizing our bodies (so no ripped jeans, no tight clothing, no leather). Basically women should always try to wear a skirt, pants only if necessary for the situation or work, men should always wear a nice collared shirt, not tight nor baggy jeans.
Some elder/CO type people go as far as saying that basically you should wear things that are only slightly more comfortable than when you go to a meeting. I know people, ex-Bethelites that basically wore suits to the beach, only leaving behind their jacket and tie.