Hm. After thinking about this a little, perhaps there is a way to have cake and 
consume it justly.
Since `ss` is now being used to check ports, I suggest a postinst check with 
`ss -6` to see if the icmp6 entry exists or not. If it does not exist, write a 
marker file out on disk that ipv6-appears-disabled. Check the marker during 
each postinst and select between the two "default" conditions automatically. 
Duplicate the `sites-available/default` with an additional copy 
`sites-available/default-ipv6`, with the inverse listen commented out, and the 
appropriate sites-enabled symlink. If either symlink is removed, the postinst 
doesn't recreate it.

This way `default` remains true to upstream, while the debian/ubuntu
ipv6 listen addition is obvious to any operators. A quirk like this
could easily be pointed out in the focal release notes as an ubuntuism.

As far as I am aware, during deployments of nginx by site operators it
is common to remove the `sites-enabled/default` symlink to disable it
without apt freaking out about 3 way merges.

Currently, I don't even utilize the sites-available or sites-enabled at all 
with my ISO spins.
The last line in my `/etc/nginx/conf.d/autoconfig.conf` is:

`include /opt/project/*/resources/latest/config/http/*/*.conf;`

It would be no trouble at all for me to add the scripted removal of the
`sites-enabled/default-ipv6` symlink alongside my existing scripted
removal of `sites-enabled/default`.

Means a slightly mungier postinst; but appropriate comments detailing
the idea there would go a long way.

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https://bugs.launchpad.net/bugs/1743592

Title:
  NGINX fails to start/install/upgrade if IPv6 is completely disabled.

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