A noted by brooks@ here[1], we could probably remove the code which assisted with transitioning from the *.sh style of rc service scripts to the currently used ones. It's been over 20 years and it might be a good time to remove remaining bits. That would involve both the rc code and its documentation. We should also check the doc tree. [1]: https://reviews.freebsd.org/D47264?id=145385#inline-283033
It looks like nextboot_enable has been removed from defaults/loader.conf in https://cgit.freebsd.org/src/commit/?id=e307eb94ae520d98dc1d346a0c53667a41beab5d. I'm CCing the author of that commit. Maybe the issue described here has been overcome by events.
(In reply to Mateusz Piotrowski from comment #1) Whoops, wrong PR. Sorry.