Otherwise the following messages would be printed by ntpd:
Sep 2 05:18:21 localhost ntpd[15849]: restrict default: KOD does nothing without LIMITE.
Sep 2 05:18:21 localhost ntpd[15849]: restrict ::: KOD does nothing without LIMITED.
Debian uses the same set of "restrict" keywords (see:
https://sources.debian.org/src/ntp/1:4.2.8p13+dfsg-2/debian/ntp.conf).
* gnu/services/networking.scm (ntp-shepherd-service): Add the 'limited'
keyword to both the IPv4 and IPv6 'restrict' directives.