Almost 9 years ago we set up some publicly accessible IPv6 time servers. At the time there were few to no options for NTP over IPv6.
However, it’s time to retire those servers. They’ve run faithfully for almost 10 years and their watch is over. We don’t have a replacement for this service, but NIST has IPv6 NTP servers in their pool: https://tf.nist.gov/tf-cgi/servers.cgi