On IOS routers you can disable built-in DHCP server issuing command
no service dhcp
you should expect that it will disable whole DHCP service no matter what protocol it’s going to service. Wellm you are wrong. It disable only IPv4 DHCP service leaving IPv6 one still running. Therefor following configuration
ipv6 dhcp pool IPv6 dns-server FC00:2::D911:220A ! interface GigabitEthernet0/0 ipv6 address FC00:3::1/64 ipv6 nd other-config-flag ipv6 dhcp server IPv6
will still work and built-in DHCP server will assign DNS information to requesting IPv6 host.