One of the people in an IRC channel I frequent pointed out a site I've been building uses CDNs that are IPv4 only. I never realized this, I just assumed every major provider had deployed IPv6. Oh, how very wrong I was. A quick check of some major (to me) sites shows a shocking lack of IPv6, including:

• Bootstrap (stackpath.bootstrapcdn.com)
• Discord
• FontAwesome (use.fontawesome.com)
• GitHub/GitHub pages
• GitLab/GitLab pages (self-hosted supports IPv6, but officially hosted GitLab only supports IPv4 due to Azure limitations)
• jQuery, IF you use code.jquery.com (some tutorials use ajax.googleapis.com, which does have IPv6, but an unfortunate amount use code.jquery.com, including the getting started page for Bootstrap)
• Parts of Amazon/AWS (Amazon is IPv4 only, some of AWS is IPv4 only, including S3)
• Reddit
• Stack Overflow/Exchange/etc
• Twitter

An honorable mention goes to Angular's websites because the websites themselves are IPv4 only but the libraries are hosted on ajax.googleapis.com, which is IPv6 accessible. I checked npm, PyPI, RubyGems, and Tildes, and they all support IPv6.

I can understand why companies like Amazon have partial support (upgrading can be a PITA if you're a cloud service provider with uptime requirements), but then you have services like Discord (launched in 2015 with no obligation to maintain service) that only support IPv4. At the very least, I'd expect CDNs referenced by thousands (if not millions) of webpages to be on IPv6 by now.

Am I missing something? CDNs are pretty static, it's just a matter of choosing one that supports IPv6, you don't even need to update your application if you just change the DNS entries.