Re: IPv6 should have taken off by now
I read the document. On v6, you are nominally expected to receive a /48, which gives you 16 bits (4 hex characters) to play with for your subnetting. Y.IPv6RefModel is mostly just a suggestion on how to use those 16 bits.
The suggestion boils down to: use the first character for a site ID, the second character for a category ID (DMZ/servers/LAN/IoT/other) and the third and fourth for a subnet number. It then says that if you set the site ID to 0 and make sure the 4th hex character is also 0, the subnet plan only uses 8 bits which makes it small enough to reasonably insert into a v4 address (since it then only needs a /16 rather than a /8).
It's not explained well in the recommendation, but if you use the same subnetting plan for v4 and v6 and are careful to also use the same host IDs for dual stack devices (which involves limiting yourself to only the first 256 addresses of the v6 /64...) then it's possible to define a stateless translation rule between v4 and v6 IPs for those devices. However, the mapping is purely local for the given network and there's no way to tell random people on the internet about the rule, so any translation would have to be done on your own local NAT64 router. There's no arbitrary v4<->v6 tunneling here.
Note that none of this is fundamentally new. You could already do all of it; this is just a recommendation for one way of going about it.
As for my opinion on the recommendation... it's coherent and it looks like it achieves the goals it sets out to achieve, but I'm not convinced the goals make much sense. Most people already have a v4 network with an incompatible addressing plan, so you can only really use this on a greenfield network. But if you have a greenfield network then there are better ways of going about it without paying the costs of this plan.