The NTP pool project is participating in World IPv6 day. Well, not participating in the 'switching off after 24 hours' bit: 2.pool.ntp.org now has AAAA records and will keep them. In the current default configuration this means a system with broken IPv6 connectivity will still work, it will just decide one of the servers is unreachable and tick on happily.koos@greenblatt:~$ host 2.pool.ntp.org 2.pool.ntp.org has address 22.214.171.124 2.pool.ntp.org has address 126.96.36.199 2.pool.ntp.org has address 188.8.131.52 2.pool.ntp.org has IPv6 address 2001:4f8:fff7:1::17 2.pool.ntp.org has IPv6 address 2607:f128:42:63::2 2.pool.ntp.org has IPv6 address 2600:3c00::2:b401Source: Experimentally enabling IPv6 - NTP Pool News.
It's not possible to add ntp.cs.uu.nl to the ipv6 pool at the moment, but ntp.idefix.net is reachable via IPv6 and is ok as a server. So it is in the pool.