I think you are having a very romanticized view of 6to4 and its derivatives.
By the late 2000s it was clear as day that 6to4 wasn't working out (the design rationales of contemporary IPv4<->IPv6 transition technologies will tell you why [0]). By extension, AutoIPv6 which was building on 6to4 was also unlikely to work out. Even worse, AutoIPv6 relies at least partially on anycast 6to4 which was later deprecated due to operational problems [1].
The only surviving 6to4 derivative is 6rd and even that is mostly phased out.
>As for current v6 being interoperable with v4, that is patently false.
You need to define your scope. Are you talking about programming? Or ability for IPv6 clients to talk to IPv4 servers? Or...?
IPv6 is interoperable with IPv4 from a programmer's perspective, once you upgrade your sockets from accepting sockaddr_in to accepting sockaddr_in6, your program automatically receives both IPv4 and IPv6 packets with IPv4 addresses represented as ::ffff:x.y.z.w.
And from a client's perspective, IPv6 clients absolutely can connect to IPv4 servers through a border relay of some sort, typically NAT64. But you can do SIIT as well if you are feeling fancy. Note that this is not much different from 6to4 and its derivatives.
As for the perspective of a server, indeed this is unsolved but again AutoIPv6 doesn't solve the issue as well since the server still needs a public IPv4 address.
By the late 2000s it was clear as day that 6to4 wasn't working out (the design rationales of contemporary IPv4<->IPv6 transition technologies will tell you why [0]). By extension, AutoIPv6 which was building on 6to4 was also unlikely to work out. Even worse, AutoIPv6 relies at least partially on anycast 6to4 which was later deprecated due to operational problems [1].
The only surviving 6to4 derivative is 6rd and even that is mostly phased out.
>As for current v6 being interoperable with v4, that is patently false.
You need to define your scope. Are you talking about programming? Or ability for IPv6 clients to talk to IPv4 servers? Or...?
IPv6 is interoperable with IPv4 from a programmer's perspective, once you upgrade your sockets from accepting sockaddr_in to accepting sockaddr_in6, your program automatically receives both IPv4 and IPv6 packets with IPv4 addresses represented as ::ffff:x.y.z.w.
And from a client's perspective, IPv6 clients absolutely can connect to IPv4 servers through a border relay of some sort, typically NAT64. But you can do SIIT as well if you are feeling fancy. Note that this is not much different from 6to4 and its derivatives.
As for the perspective of a server, indeed this is unsolved but again AutoIPv6 doesn't solve the issue as well since the server still needs a public IPv4 address.
[0]: https://en.wikipedia.org/wiki/IPv6_rapid_deployment#Comparis...
[1]: https://datatracker.ietf.org/doc/html/rfc6343#section-3