Comment 34 for bug 1821867

@ sil2100:
Yes, that's what I did - I used proposed and indicated it by the following kernel arg (mentioned above):
apt-setup/proposed=true

Since I still have the systems up and running, here are more detailed apt outputs (for all 3 affected releases):

ubuntu@hwe0007:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 19.04
Release: 19.04
Codename: disco
ubuntu@hwe0007:~$ apt-cache policy netplan.ionetplan.io:
  Installed: 0.97-0ubuntu1~19.04.1
  Candidate: 0.97-0ubuntu1~19.04.1
  Version table:
 *** 0.97-0ubuntu1~19.04.1 500
        500 http://us.ports.ubuntu.com/ubuntu-ports disco-proposed/main s390x Packages
        100 /var/lib/dpkg/status
     0.96-0ubuntu4.1 500
        500 http://us.ports.ubuntu.com/ubuntu-ports disco-updates/main s390x Packages
     0.96-0ubuntu4 500
        500 http://us.ports.ubuntu.com/ubuntu-ports disco/main s390x Packages

ubuntu@hwe0006:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.10
Release: 18.10
Codename: cosmic
ubuntu@hwe0006:~$ apt-cache policy netplan.io
netplan.io:
  Installed: 0.97-0ubuntu1~18.10.1
  Candidate: 0.97-0ubuntu1~18.10.1
  Version table:
 *** 0.97-0ubuntu1~18.10.1 500
        500 http://us.ports.ubuntu.com/ubuntu-ports cosmic-proposed/main s390x Packages
        100 /var/lib/dpkg/status
     0.96-0ubuntu0.18.10.3 500
        500 http://us.ports.ubuntu.com/ubuntu-ports cosmic-updates/main s390x Packages
     0.40.2 500
        500 http://us.ports.ubuntu.com/ubuntu-ports cosmic/main s390x Packages

ubuntu@hwe0005:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.2 LTS
Release: 18.04
Codename: bionic
ubuntu@hwe0005:~$ apt-cache policy netplan.io
netplan.io:
  Installed: 0.97-0ubuntu1~18.04.1
  Candidate: 0.97-0ubuntu1~18.04.1
  Version table:
 *** 0.97-0ubuntu1~18.04.1 500
        500 http://us.ports.ubuntu.com/ubuntu-ports bionic-proposed/main s390x Packages
        100 /var/lib/dpkg/status
     0.96-0ubuntu0.18.04.4 500
        500 http://us.ports.ubuntu.com/ubuntu-ports bionic-updates/main s390x Packages
     0.40.1~18.04.4 500
        500 http://ports.ubuntu.com/ubuntu-ports bionic-security/main s390x Packages
     0.36.1 500
        500 http://us.ports.ubuntu.com/ubuntu-ports bionic/main s390x Packages