If you put part of the address in the body space, you can't encrypt the entire body.

IPv6 adoption has been linear for the last two decades. Currently, 48% of Google traffic is IPv6.[1] It was 30% in 2020. That's low, because Google is blocked in China. Google sees China as 6% IPv6, but China is really around 77%.

Sometimes it takes a long time to convert infrastructure. Half the Northeast Corridor track is still on 25Hz. There's still some 40Hz power around Niagara Falls. San Francisco got rid of the last PG&E DC service a few years ago. It took from 1948 to 1994 to convert all US freight rail stock to roller bearings.[2] European freight rail is still using couplers obsolete and illegal in the US since 1900. (There's an effort underway to fix this. Hopefully it will go better than Eurocoupler from the 1980s. Passenger rail uses completely different couplers, and doesn't uncouple much.)[3]

[1] https://www.google.com/intl/en/ipv6/statistics.html

[2] https://www.youtube.com/watch?v=R-1EZ6K7bpQ

[2] https://rail-research.europa.eu/european-dac-delivery-progra...