Plex can work over IPv6, but it all has to be configured manually.
Here’s how to do it: Ipv6 support for myplex - #269 by thornrag
Don’t forget to add a rule in the IPv6 firewall of your router to allow incoming connections on TCP port 32400 towards your server.
Also bear in mind that when you are serving only over IPv6, the client also needs to have IPv6 connectivity itself - an IPv4 client cannot connect to an IPv6 server.
