You need to add “includeIPv6=1” to your parameter list in your URL:
https://plex.tv/api/resources?includeIPv6=1&includeHttps=1&X-Plex-Token=
Otherwise no IPv6 addresses will be shown at the resources endpoint.
However, there are very few (if any) current Plex clients which support connections via IPv6. Apple devices used to, but that functionality has not been added back to the new experience apps yet. The possible exception to this is the Apple TV; the legacy app is still available there.
I’m pretty sure there’s a feature suggestion floating around requesting full IPv6 support across the platform; head to the Feature Suggestions area of the forums and search for “IPv6” and it should turn up as a result.
