Hello,
I had this reported downstream (Devuan) and it was also on supervision list:
https://dev1galaxy.org/viewtopic.php?id=6874
https://skarnet.org/lists/supervision/0677.html

The report on supervision says it kills PID 1 and panic, according to my testing on Debian if I press sysrq-k on getty-tty1 it kills runsvdir and all runsv childs; runit enters stage 3 (I see messages fail:runsv not running and then shutdown tasks are run) and shutdown the system.

Laurent suggests to discard stdin, tried this in stage 2 before runsvdir exec, but it doesn't work; If both stdin and stdout are discarded,
sysrq-k only kills the getty which is then restarted, so it behaves as expected, but all runsvdir output is lost.

Is there a way to address this in runsvdir.c?