Fixing "Stopping remaining crypto disks..." in SysVinit Debian and Devuan

SysVinit Debian/Devuan installations have this problem: encrypted disks are not properly unmounted during shutdown, which results in this:

In the end, after some timeout, it just fails with an error.

Quite famous bug, it seems:

But we will not tolerate it, will we?

Run vgdisplay and lvdisplay.

# vgdisplay
  --- Volume group ---
  VG Name               debian-vg
  System ID             
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  3
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                2
  Open LV               2
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               19.74 GiB
  PE Size               4.00 MiB
  Total PE              5054
  Alloc PE / Size       5054 / 19.74 GiB
  Free  PE / Size       0 / 0   
  VG UUID               H3PpKg-f8BB-MncA-qJXb-Alf1-k80E-66KB3l

# lvdisplay
  --- Logical volume ---
  LV Path                /dev/debian-vg/root
  LV Name                root
  VG Name                debian-vg
  LV UUID                TVTUmj-DAeh-3RE2-FIEV-ADRd-Kms7-ObWMIO
  LV Write Access        read/write
  LV Creation host, time debian, 2020-01-01 18:53:41 -0500
  LV Status              available
  # open                 1
  LV Size                18.74 GiB
  Current LE             4798
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:1

  --- Logical volume ---
  LV Path                /dev/debian-vg/swap_1
  LV Name                swap_1
  VG Name                debian-vg
  LV UUID                ASGoEn-2yn4-yQoX-aXS3-fEf2-oFca-WKUNqJ
  LV Write Access        read/write
  LV Creation host, time debian, 2020-01-01 18:53:41 -0500
  LV Status              available
  # open                 2
  LV Size                1.00 GiB
  Current LE             256
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           254:2

Now go to /lib/cryptsetup and open the cryptdisks-functions file. Find do_stop function and add these commands at the beginning:

lvchange -an /dev/debian-vg/root
vgchange -an debian-vg
dmsetup remove_all

Save the file and it's done.