For IPv6 in both Horizons and Odyssey, if you have edited your "AppConfigLocal.xml" to turn it on, you should see the appropriate IPv6 values in your game's network tab on the main menu.
Also know, that the IPv6 protocol and setting in Elite is only used "to directly connect you with other players who have IPv6 enabled". It states this in the main menu/network settings when you put your mouse over IPv6 and look at the right panel. I have no clue whether IPv6 is used by Elite for other network traffic in or out of the game.
Here are the settings to add to the file mentioned above.
<?xml version="1.0" encoding="UTF-8" ?>
<AppConfig>
<Network
Port="5100"
IPv6="1"
IPv4="1"
IPv6Port="5100"
ReportSentLetters="1"
ReportReceivedLetters="1"
VerboseLogging="1"
UseNicGuid="{68D6A5CE-2D43-4E09-A213-BBD6D56E1710}" />
<Journal Address="0.0.0.0" Port="0" />
</AppConfig>
I use a port forward on my router to use port 5100 (in blue) on the IPv4 protocol in the main menu/network settings as well as using the same port number (in yellow) for the IPv6 address. For IPv4, this works. Whether a port setting in IPv6 is used or useful, I do not know, but Frontier would not have included the setting if it was not useful.
When properly set (including your router settings for IPv4 <(Port Forward to 5100) and LAN/WAN IPv6 Enabled (with the router's IPv6 DHCP Server turned OFF because it's Not needed) you should get a very reliable connection the the Frontier Servers.
Also note, IPv6 is BLOCKED by default if you use a VPN connection to connect to the internet because VPN does not support IPv6 connections. VPN was designed for the IPv4 network protocol only. So, even if you have IPv6 turned on in Elite and your router configured properly, if you connect via a VPN connection to the internet, IPv6 will not work beyond your local network.
So, don't run Elite through a VPN connection if you plan to use IPv6 to instance with other players.
Hope this helps.