Comment 2 for bug 1872941

in the function _set_url_paths it makes up the defaults:

  # generic
  url_prefix = "current/images"
  ...
  # s390x specific as it can't find "normal" kernel/initrd there
  hvmroot = "%s/generic/" % url_prefix
  kernel_basename = "kernel.%s" % self._debname

That will construct
http://ports.ubuntu.com/ubuntu-ports/dists/bionic/main/installer-s390x/current/images/generic/kernel.ubuntu

Which was the correct old path.
The new path would be the new path.

http://ports.ubuntu.com/ubuntu-ports/dists/focal/main/installer-s390x/current/legacy-images/generic/kernel.ubuntu

But that differs based on the version, if you want to install a bionic guest the old code works.
@Andrew - you might want to ask to keep the old layout instead of changing this (and probably many other tools/scripts)?