What hardware, especially ASICs, do not support wire speed IPv6 and have not for a decade or two?
T-Mobile was gave a presentation on going IPv6-only in 2017:
> For the past 10 years T-Mobile has worked towards creating an IPv6 environment and we are now getting very close to our goal. Stephan presents learning on how to successfully enable IPv6-only using DNS64 with or without 464XLAT. He will do a live demo of the different IP interfaces on an Android handset. Finally, he will discuss and give some best practices on how to handle DNS, applications, and websites that are having issues with DNS64.
Any hardware router introduced in the last two decades has IPv6 support; in my somewhat outdated experience the barrier to implementation is implementing dual-stack logic in software built for single stack.
ISPs in Belgium implemented IPv6 a decade ago (some even two decades). ISPs elsewhere could have done it too by now (nobody is still using 10+yo hardware, I hope...).
You also have to deploy new DNS code to handle a new record type to handle longer "IPv4+" addresses.
You also have to deploy new OS and library code with new socket, etc, APIs because all in_addr_t definitions and data structures are 32-bit-only.