Support: https://lists.freedesktop.org/mailman/listinfo/systemd-devel
Documentation: man:journald.conf(5)

A service has logged too many messages within a time period. Messages
from the service have been dropped.

Note that only messages from the service in question have been
dropped, other services' messages are unaffected.

The limits controlling when messages are dropped may be configured
with RateLimitIntervalSec= and RateLimitBurst= in
/etc/systemd/journald.conf or LogRateLimitIntervalSec= and LogRateLimitBurst=
in the unit file. See journald.conf(5) and systemd.exec(5) for details.