6-15 and 6-20 receptacles have always been common to serve window shakers. I never heard they were illegal but the TR and AFCI rules may have made them so if it expanded to 240v circuits.
Originally most of the GFCI and AFCI rules only affected 15 & 20a 120v circuits.
What are they changing?
To be honest, since I have been retired about 25 years, I have lost track of a couple cycles. I was staying current with my code change CEUs up until around 2014.
