I've written a similar CLI tool (not a website), netq [1]. I believe it can be an alternative to some of the functionalities of the website.

If one is able to get a public IPv6 from a public IP finder service, I guess that means his machine is able to access the IPv6 internet.

  netq -6 p 
  netq -4 p

Some finders report ISP too (use -r).

Also, kudos to jfesler for his works on maintaining the website through the years.

[1]: https://github.com/pvonmoradi/netq