Going IPv6 Only
The need for IPv6 support is real!This site: https://whynoipv6.com/country/us -- provides a "wall of shame" summary of sites that do not yet fully support IPv6, in order of Alexa rank. It is definitely surprising, even shameful, that so many top sites make this list.That Twitter and Amazon don't have full IPv6 support yet blows my mind.It's also worth noting that IPv6 networks definitely DO exist in the real world.My tiny startup provides software to run sports teams (mostly swim teams) and we've run into several cases where new wifi gets installed at a neighborhood pool, and the network is IPv6 only. So far, the pattern seems to be cases where AT&T is the ISP in the Houston area. To troubleshoot these issues via phone support, we ask if the customer can reach amazon.com or twitter.com. If not, it's a pretty good sign they are on a IPv6 only network.Apple also requires IPv6 support for backend services for approval of iOS apps in the App Store (although, in practice, this requirement is not consistently checked).
See: https://developer.apple.com/support/ipv6/Fortunately, adding IPv6 support to a site is relatively simple using Cloudflare. 
See: https://www.cloudflare.com/ipv6/[Edit] To clarify, I'm not affiliated with the Why No IPv6? site. I just found it to be a useful resource. The site credits https://crawler.ninja/ as the source of its data.[Edit 2] Added link to Apple's IPv6-only support policy.