It got disabled in 2003:
commit acb18acfec
Author: Poul-Henning Kamp <phk@FreeBSD.org>
Date: Sun Feb 23 18:09:05 2003 +0000
Bracket the kern.vnode sysctl in #ifdef notyet because it results
in massive locking issues on diskless systems.
It is also not clear that this sysctl is non-dangerous in its
requirements for locked down memory on large RAM systems.
There does not seem to be practical use for it and the disabled routine
does not work anyway.
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D39127
62a573d953