Building codes have historically focused on life safety. Sustainability and energy efficiency of residential structures have worked their way into the codes over time; the requirements get progressively more demanding with each code cycle.

I don't know what building code applies in the OP's location. The International Residential Code (IRC) forms the core of most statewide building codes. The duct insulation requirements in the 2018 IRC vary depending on both the size and location of ducts. R8 insulation is required only for ducts in attics and only if they are greater than 3" in diameter. For ducts less than 3" diameter in attics, R6 insulation is required. For other areas in a home, ducts need to be insulated with R6 insulation only if they are larger than 3" in diameter and R4.2 if they are less than 3" in diameter.

Going beyond code requirements for insulating ducts can be difficult because higher R-value insulation may not be readily available as the OP's experience suggests.

When did it become a crime to order duct and have it shipped across state lines?

Larger operators who do mail order don't ship products into locations where that product doesn't meet local requirements.

If you're in an area where federal rules require a higher SEER standard than elsewhere, as an example, you won't be able to get the impermissible equipment from a distributor nor would a contractor install it.

Where I am, more energy efficient lightbulbs are required for new purchases and less efficient types can't be sold that are sold elsewhere. I can find such other ones on Amazon, or Home Depot, or any of the lightbulb selling websites that are okay elsewhere. When I try to order them, the message comes up "this product can't be shipped to the stated address on record (or something like that)." Same is true for other things that in my area that by state law much be at a higher standard that what' required elsewhere.

I'm not saying you cant ship R8 to locales where it is not required by code, just that installers don't use it if it's not code, so their suppliers (stores) won't keep it in stock.

I'm not sure there are any localities where R8 insulation isn't required for ducts larger than 3" installed in attics.

I can tell you that pre-insulated flex duct with R8 insulation was one of a number of HVAC components in short supply in our area during the first year of the pandemic.

I'm not sure about this detail. However, there are a lot of platforms that let you know these types of details.