Comment 32 for bug 1865218

I had also wondered if --allow-third-party (or its corresponding env var RELEASE_UPGRADER_ALLOW_THIRD_PARTY) would be worth trying, however in looking at the do-release-upgrader code, in DistUpgradeController::rewriteSourcesList() it looks like it's used for skipping the mirror check. So if you're an admin with a local mirror of the Ubuntu repositories this would allow use of your mirror when your users are upgrading. Something like that.

Handling of proposed is special cased separately in DistUpgradeController::rewriteSourcesList():

            # Disable proposed on upgrade to a development release.
            if (not entry.disabled and self.options
                and self.options.devel_release == True and
                "%s-proposed" % self.fromDist in entry.dist):
                logging.debug("upgrade to development release, disabling proposed")
                entry.dist = "%s-proposed" % self.toDist
                entry.comment += _("Not for humans during development stage of release %s") % self.toDist
                entry.disabled = True
                continue

I had some thoughts on how this could be modified to support a cli param + env var like --allow-third-parties, but I'll write those notes for the bug Simon opened, LP: #1880760.