On 14-CURRENT as well as 13.0-RELENG and 13-STABLE (tested), /etc/rc.conf (or the default in /etc/defaults/rc.conf) provides only the entries
for the IPv6 router advertisement daemon rtadvd(8). Man page of the rtadvd offers some options which can be provided to the rtadvd, but there is no chance to offer such options via a variable like "rtadvd_flags" to override the default, i.e. for debugging purposes as we see this for a couple of other rc-variables (see man rc.conf).
It would be nice to have a flag variable.