i would like to try to help about this
NTP pool connectivity study 2017
The objective of this study of the NTP pool project is to analyze its reachability over IPv4 and IPv6.
The results show that of all DNS region zones, 35.29% provide IPv4 A records and 5.25% provide IPv6 AAAA records.
According to this only 14.88% of all active DNS region zones are reachable using IPv6 connectivity.
Because the use of this protocol is growing fast, it is probably recommended to increase its IPv6 reachability.
The results were obtained using automated queries to NTP Online Tester.