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-...