SIDN Labs is the research arm of the .nl domain registry, writing about DNS, DNSSEC, routing and internet infrastructure.
HN commenters, including a former colleague, link it for background on DNS experiments and proposals like IPv6-only testing and DNS Resolution Required.
My former colleague Marco Davids from SIDN Labs (the R&D department at the .nl TLD operator) did an experiment in 2021 where he actively disabled IPv4 support on all components in his test network, even disabling the complete IPv4 stack in the FreeBSD kernel (not possible on Linux, at least not at the time). So far, his test is the only thing I know of that came close to an authentic simulation of an IPv6-only world.
https://www.sidnlabs.nl/en/news-and-blogs/can-we-do-without-ipv4-yet-news-of-our-annual-ipv6-only-experiment
This is very very similar to something developed by SIDN labs called "DNS Resolution Required", you can read the blog post here:
https://www.sidnlabs.nl/en/news-and-blogs/dns-resolution-required-can-help-make-the-internet-safer
That's not the whole story.
Read our blog for the motivation behind it:
https://www.sidnlabs.nl/en/news-and-blogs/a-digital-for-sale-sign-for-nl-domain-names