I just saw:
System shutdown time has arrived
About to run shutdown hooks...
Stopping cron.
Stopping inetd.
Saved entropy to /var/db/entropy-file.
Forcibly unmounting tmpfs filesystems
Removing block-type swap devices
swapctl: removing /dev/ld0b as swap device
Mon Oct 22 17:28:15 BST 2018
Done running shutdown hooks.
[ 9677.6527895] panic: kernel diagnostic assertion "error == EOPNOTSUPP" failed: file "/usr/src/sys/kern/vfs_vnode.c", line 984
[ 9677.6639354] cpu0: Begin traceback...
[ 9677.6639354] 0x80cb7d0c: netbsd:db_panic+0xc
[ 9677.6744202] 0x80cb7d24: netbsd:vpanic+0x138
[ 9677.6744202] 0x80cb7d3c: netbsd:kern_assert+0x40
[ 9677.6844490] 0x80cb7d74: netbsd:vrevoke_suspend_next+0x98
[ 9677.6944748] 0x80cb7dac: netbsd:vrevoke+0x2c
[ 9677.7044915] 0x80cb7dbc: netbsd:genfs_revoke+0x20
[ 9677.7147293] 0x80cb7ddc: netbsd:VOP_REVOKE+0x3c
[ 9677.7147293] 0x80cb7e3c: netbsd:exit1+0x868
[ 9677.7247581] 0x80cb7e64: netbsd:sigexit+0x150
[ 9677.7359090] 0x80cb7ed4: netbsd:postsig+0x268
[ 9677.7359090] 0x80cb7f14: netbsd:lwp_userret+0x1bc
[ 9677.7463008] 0x80cb7f34: netbsd:userret+0x94
[ 9677.7563186] 0x80cb7fac: netbsd:syscall+0x204
[ 9677.7563186] cpu0: End traceback...
[ 9677.7689505] dump to dev 92,1 not possible
[ 9677.7736334] rebooting...
Lack of dump, so not much to go on...