IPv6 doesn't exist automatically, either. If an ISP is going to move their customers to it without setting up the requisite transition mechanisms, I don't see how that's any different than failing to meet the myriad requirements for provisioning a functional IPv4-only network in the modern age. Whom do you blame when an ISP forgets to configure BGP on their routers?

You might think such a scheme is "requisite", but the fact remains that there is insufficient incentive for the ISPs to do them, and they plainly don't do them, as my cited example shows.

You can assign blame however you like, but it won't help your IPv6 customers/users to reach your servers.

I really don't know what your example shows, because I don't read Swedish, and the Google translate version is predictably mangled and difficult to parse. To the extent it says anything, it's very vague and seems to recount a single, unique incident. This and your apparent inability to provide case studies in English leads me to the belief that the problem is rare.

I'll also add that I've been in the employ of a company that does business exclusively in Asia for years. We've as yet not encountered this problem.

If you have experience with Asian ISPs, could you then maybe report on how common it is for them to have IPv6-only versus IPv6-plus-transition-mechanism?

This data would be better than my anecdotal story about an unnamed company, so please provide any you have.

In my experience? Exactly 0% of ISPs are strict IPv6-only. That doesn't mean there isn't one anywhere, we don't have millions of users throughout the continent, mostly Taiwan and (to a lesser extent) China. But we've yet to find one.

When I say the issue has not arisen, I mean it. We have two infrastructures, one is basically 8 years old, the other is about three. At the time the latter was setup, our provider wasn't even ready to deploy IPv6 widely, they were still learning and testing with a handful of select customers.

Both legacy and modern run solely on IPv4, and we've not had cause to revisit the issue.

When we get reports of network problems not attributable to simple user error, the cause is almost always either misconfigured routers breaking path MTU discovery, or ridiculously strict firewall rules that don't even allow port 443 out.

I don't even know how many of our users have IPv6 at all. I have no reason to know nor any good way to measure it short of calling all the ISPs or sending people to our customer's homes to check.

DS-Lite and NAT64 definitely do exist and any ISP that drops IPv4 with no transition mechanism is going to be really broken.