Corey,

Relation data (enable-dvr: "True"): https://paste.ubuntu.com/p/zPs3kB8t4d/

ubuntu@adze:~$ sudo grep -RiP python3-neutron-fwaas /var/log/juju/ ; echo $?
1
ubuntu@adze:~$ grep -RiP python3-neutron-fwaas /var/log/apt/ ; echo $?
1


What can be seen below is that python3-neutron-fwaas is not installed but neutron-l3-agent depends on it while the python2 version is gone.

So you are right about the lack of python3-neutron-fwaas.

-----

term.log:

update-alternatives: using /usr/bin/python3-neutron-l3-agent to provide /usr/bin/neutron-l3-agent (neutron-l3-agent) in auto mode

Purging configuration files for python-neutron (2:13.0.2-0ubuntu3.1~cloud0) ...
Purging configuration files for neutron-l3-agent (2:13.0.2-0ubuntu3.1~cloud0) ...
Purging configuration files for python-neutron-fwaas (1:13.0.1-0ubuntu1~cloud0) ...

history.log:

Start-Date: 2019-05-08  22:52:31
Commandline: apt-get --assume-yes purge python-neutron python-nova
Purge: neutron-l3-agent:amd64 (2:13.0.2-0ubuntu3.1~cloud0), python-nova:amd64 (2:18.1.0-0ubuntu1~cloud0), python-neutron:amd64 (2:13.0.2-0ubuntu3.1~cloud0), python-neutron-fwaas:amd64 (1:13.0.1-0ubuntu1~cloud0)
End-Date: 2019-05-08  22:52:40

ubuntu@adze:~$ apt policy python3-neutron
python3-neutron:
  Installed: 2:13.0.2-0ubuntu3.1~cloud0
  Candidate: 2:13.0.2-0ubuntu3.1~cloud0
  Version table:
 *** 2:13.0.2-0ubuntu3.1~cloud0 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu bionic-updates/rocky/main amd64 Packages
        100 /var/lib/dpkg/status
ubuntu@adze:~$ apt policy neutron-l3-agent

neutron-l3-agent:
  Installed: (none)
  Candidate: 2:13.0.2-0ubuntu3.1~cloud0
  Version table:
     2:13.0.2-0ubuntu3.1~cloud0 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu bionic-updates/rocky/main amd64 Packages
     2:12.0.5-0ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
     2:12.0.1-0ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages



ubuntu@adze:~$ apt-cache depends neutron-l3-agent
neutron-l3-agent
  Depends: conntrack
  Depends: iputils-arping
  Depends: keepalived
  Depends: neutron-metadata-agent
 |Depends: python-neutron
  Depends: python3-neutron
 |Depends: python-neutron-fwaas
  Depends: python3-neutron-fwaas
  Depends: radvd

ubuntu@adze:~$ apt policy python3-neutron-fwaas
python3-neutron-fwaas:
  Installed: (none)
  Candidate: 1:13.0.1-0ubuntu1~cloud0
  Version table:
     1:13.0.1-0ubuntu1~cloud0 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu bionic-updates/rocky/main amd64 Packages

ubuntu@adze:~$ apt policy python-neutron-fwaas
python-neutron-fwaas:
  Installed: (none)
  Candidate: 1:13.0.1-0ubuntu1~cloud0
  Version table:
     1:13.0.1-0ubuntu1~cloud0 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu bionic-updates/rocky/main amd64 Packages
     1:12.0.1-0ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
     1:12.0.0-0ubuntu1 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages
     1:10.1.0-0ubuntu1~cloud0 500
        500 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-proposed/ocata/main amd64 Packages