Following fork will handle updates of IPv6 addresses. https://github.com/geraldhansen/ddclient
As ddclient is just on file, just replace them with the one from the fork
With the new version you have the option ipv6, which can be used on the command line ddclient -ipv6 or with ipv6=yes in the configuration file. With this change ddclient tries to use IPv6 connections and use also the IPv6 addresses for the updates.
My pull request was accepted and merged into the main branch. The official ddclient version can handle IPv6 with it now.
Resources:
[1] https://github.com/geraldhansen/ddclient
[2] https://myonlineportal.net/howto_dyndns_ddclient_on_raspberrypi#raspberrypi_ipv6
[3] https://github.com/wimpunk/ddclient