While definitely not a startup, the National Archives made https://clintonwhitehouse1.archives.gov/ and https://clintonwhitehouse2.archives.gov/ IPv6-only.

While I don't think a couple administration's website archives are enough to drive adoption, one could imagine there might be some government resources that might.

Sadly browsers don't seem to warn users that they couldn't connect because of the lack of IPv6 (and doing so would be difficult for IPv6-only DNS servers), so it just looks like a regular connection failure.