Comment 3 for bug 1890177

One easy fix would possibly be the following:

# debian/rsyslog.postinst
case "$1" in
    configure)
        adduser --system --group --no-create-home --quiet syslog || true
        adduser syslog adm || true
       +adduser syslog tty || true

I have tested it in a PPA, and it works just fine:

Preparing to unpack .../rsyslog_8.2001.0-1ubuntu1+test2020307b1_amd64.deb ...
Unpacking rsyslog (8.2001.0-1ubuntu1+test2020307b1) over (8.2001.0-1ubuntu1) ...
Setting up rsyslog (8.2001.0-1ubuntu1+test2020307b1) ...
The user `syslog' is already a member of `adm'.
Adding user `syslog' to group `tty' ...
==> Adding user syslog to group tty <==
Done.
Skipping profile in /etc/apparmor.d/disable: usr.sbin.rsyslogd
Processing triggers for man-db (2.9.1-1) ...
Processing triggers for systemd (245.4-4ubuntu3.1) ...

# /etc/group
tty:x:5:syslog