You can't install unofficial packages via pacman. And AFAIK none of the unofficial/AUR package managers, like yaourt, are in the official repositories so the point where you cross the line is very clear and distinct and any wiki guide to installing from the AUR makes it clear you are going into unofficial territory.

I haven't used Manjaro but they seem to intentionally hide the distinction between the official and unofficial repos, which is a bad idea.

Also, yaourt has a lot of warnings and prompts at each step along the way to make absolutely certain that you understand what you're getting yourself into with the packages you're installing. The process is certainly not for beginners. Ubuntu seems to go for a more user friendly process instead.

In ubuntu, it's even less obvious. Main, restricted, and universe are all checked together by apt, and treated the same

But main/restricted and universe are both vetted—the only people who can upload to universe are Debian developers (indirectly, via Ubuntu importing from Debian) and Ubuntu developers, and the process of becoming either of these is nontrivial.

Ubuntu's equivalent of the AUR, if I understand the AUR right, is PPAs, which are definitely not enabled by default and are fairly obvious about their third-party-ness.

(Main vs. restricted and universe vs. multiverse is just about licensing, not access control or vetting.)

It took me a good amount of googling to verify that, but you're mostly correct, there are only 132 developers with upload rights to the universe repository. Though I would argue that the distinction isn't just licensing, since Canonical themselves only support main and restricted.

Universe is Canonical's dumping ground. There are millions of vulnerable Redis instances in production today because Ubuntu doesn't feel inclined to issue an update for a major CVE affecting the redis-server package shipped for 14.04.

There's multiple layers of problems here, because as we see with Ubuntu, just because the code was built and uploaded by a trustworthy person doesn't mean it's automatically safe or secure (especially for more nefarious infections that bury themselves deep in the source tree). Remember the pwnage that brought down kernel.org for a few months? That was only a few years ago, but the infiltration had been quite serious, and if I recall correctly, there was some concern that infected code had made it into officially distributed tarballs.

In practice, I don't know that the distinction between distributing packages that are trivially exploitable and distributing packages that have exploits pre-baked in is really that big of a difference. Automated scanners often pick up and automatically exploit exposed instances within a few days.

What it boils down to is that admins need to take responsibility for their own workload and what they choose to execute, no matter the guarantees of the distributor or the claims that $Sandbox_Y is magically impenetrable (which was silly enough before, but completely farcical in a post-Spectre world).

I mean that the distinction between main and restricted and between universe and multiverse is licensing. It's a 2x2 matrix:

              supported   only community
    free       main        universe
    non-FOSS   restricted  multiverse

Oh yeah, definitely misread that