I'm not sure about regressing, but I think we've almost reached peak IPv6: everyone who planned to support IPv6 have upgraded their software and infrastructure, those that were sceptical, lazy or downright opposed to IPv6 have no plans to support it, now or ever.
Having an "ipv6" configuration option in your software, which defaults to false, falls into the latter group. (Hello, Docker!)
Having IPv6 turned off by default is not the only Docker problem with IPv6. When I tried to use IPv6 in Docker couple of years ago I found it to be unusable for my use case. I don't remember details, but remember finding tickets in github issue tracker with long discussions which went nowhere.
Having an "ipv6" configuration option in your software, which defaults to false, falls into the latter group. (Hello, Docker!)