This check was added in initial? netinet6/ import
back in 1999 (r53541).
It effectively became unnecessary after 'address/prefix clean-ups'
KAME commit 90ff8792e676132096a440dd787f99a5a5860ee8 (github) in 2001
(merged to FreeBSD in r78064) where prefix check was added to
nd6_prefix_onlink(). Similar IPv4 check (in_addroute() was added
in r137628).
Additionally, the right plance for this (or similar) check is the prefix
addition code (nd6_prefix_onlink(), nd6_prefix_onlink_rtrequest(),
in_addprefix() or rtinit()), but not the generic radix insert routine.