Also, umountroot doesn't actually unmount the root filesystem, but it remounts the filesytem root as read-only, and that should be enough to usually avoid fsck on boot-up.

And, I'm sure you wouldn't want unmounting to be done upon "S s 1", and you should restore the normal "# Default-Stop: 0 6".

It appears you are using runit rather than sysvinit? If that's the case I wonder if you have set up it's start/stop configuration to execute those scripts? I believe it's done differently from the sysvinit setup although many sysvinit scripts may be used with runit.

#! /bin/sh
### BEGIN INIT INFO
# Provides:          umountfs
# Required-Start:
# Required-Stop:     umountroot
# Default-Start:     2 3 4 5
# Default-Stop:      S s 1 0 6
# Short-Description: Turn off swap and unmount all local file systems.
# Description:
### END INIT INFO

And still nothing (filesystems won't unmount); so i installed runit, runit-helper, runit-run and runit-services in order to have runit integrated in sysv (maybe it's useless in this case, but i'll give it a try). So i read the instructions on smarden.org (https://smarden.org/runit/useinit#sysv), which say to add runsvdir-start; so i edited /etc/inittab:

#runit-sysv maintscript -- BEGIN
SV:123456:respawn:/sbin/runsvdir-start
#runit-sysv maintscript -- END

But: i couldn't find runsvdir-start in /sbin, nor in /usr/bin, /bin and /usr/sbin... Has the file been removed from runit's package?

#! /bin/sh
### BEGIN INIT INFO
# Provides:          umountroot
# Required-Start:
# Required-Stop:
# Should-Stop:       halt reboot kexec
# Default-Start:      0 6
# Default-Stop:
# Short-Description: Mount the root and /usr filesystems read-only.
### END INIT INFO

#! /bin/sh
### BEGIN INIT INFO
# Provides:          umountnfs
# Required-Start:
# Required-Stop:     umountfs
# Should-Stop:       $network $portmap nfs-common
# Default-Start:      0 6
# Default-Stop:
# Short-Description: Unmount all network filesystems except the root fs.
# Description:       Also unmounts all virtual filesystems (proc,
#                    devpts, usbfs, sysfs) that are not mounted at the
#                    top level.
### END INIT INFO

/etc/init.d/umountroot

#! /bin/sh
### BEGIN INIT INFO
# Provides:          umountroot
# Required-Start:
# Required-Stop:
# Should-Stop:       halt reboot kexec
# Default-Start:
# Default-Stop:      0 6
# Short-Description: Mount the root and /usr filesystems read-only.
### END INIT INFO

---snip---

added /etc/init.d/umountnfs.sh

#! /bin/sh
### BEGIN INIT INFO
# Provides:          umountnfs
# Required-Start:
# Required-Stop:     umountfs
# Should-Stop:       $network $portmap nfs-common
# Default-Start:
# Default-Stop:      0 6
# Short-Description: Unmount all network filesystems except the root fs.
# Description:       Also unmounts all virtual filesystems (proc,
#                    devpts, usbfs, sysfs) that are not mounted at the
#                    top level.
### END INIT INFO

I added the header for the files because they have different deps... (should stop)