Comment 1 for bug 1813923

> We do this to be able to debootstrap a fully up to date chroot.

debootstrap --arch=amd64 --components=main --include=SOME_PACKAGES --extra-suites=xenial-updates,xenial-security xenial target http://mirror

> Even if I add --no-merged-usr option (which is supposed to be the default according to the man page) the debootstrap fails.

case "$CODENAME" in
  gutsy|hardy|intrepid|jaunty|karmic|lucid|maverick|natty|oneiric|precise|quantal|raring|saucy|trusty|utopic|vivid|wily|xenial|yakkety|zesty|artful|bionic|cosmic) ;;
  *)
   # disco+ is merged-usr by default
   MERGED_USR=yes

In Focal & Jammy the opposite is true. You can change that script if you wish but Bionic is ESM only now.

However, all the debootstraps I tested with -updates and -security enabled fail and it has nothing to do with MERGED_USR.