In December 2017, I have decided to turn off the IPV4 on my computer and browse the internet in IPv6. I first noticed that most of the websites didn’t work in IPv6. I expected it. Then, I visited my own websites and discovered that embedded videos, advertisements, scripts didn’t work because their external websites were not available in IPv4. For instance, amazon.com, vimeo.com have no IPv6 support.
Then, I discovered it could raise many issues with crawling from bots. We never know the reaction of bots that would crawl my website using IPv6 and be perhaps unable to visit scripts, images, and other resources because they aren’t available through IPv6.
I have found that very few major websites accepted IPv6. On top of that, those websites don’t have embedded content from other external websites or few. There aren’t enough proof that IPv6 is not harmful to SEO because IPv6 didn’t prove its worth.