bill, the umc section 504.3.2.1 says"domestic clothes dryer moisture exhaust ducts shall be of metal and shall have smooth interior surfaces"exception allows up to 6' of approved flexible duct to connect to the metal duct.flexible portion not to be concealed.
Bill: I'm not sure how new this requirement is, sorry.
In regards to your statement about product availability, this should not surprise you. Take a trip some time to a big box (or other) hardware store and see how many non-compliant items you find. The plumbing department, for example, has many, many items that do not comply.