(this is an honest question, because when I saw this list I actually have a python script running on an apache web server that is using the cgi module to access post variables, and I never saw any need to change that, as any other "more modern" solution would be much more bothersome to configure and maintain.)
So presumably you are happy to have a web accessible endpoint running on your server that will pop a new process and start executing a local script (which might be reading/writing files, other services, etc.) without checking any permissions, enforcing any cross site scripting or CSRF protections, etc. It might be fine for the most trivial appplication. For anything that actually changes state or has side effects, I would be very concerned and it would never pass a serious security review.
* basic auth only exists at the proxy layer to protect apps with no auth.
* commonly deployed auth systems are secure without TLS?
because both are false. Check out the Rails Devise support for HTTP basic auth which is perfectly secure. And check that basically zero auth systems in the wild use PAKE and so are entirely dependent on TLS to secure the password transmission in flight.
E.g.
If mercury is in retrograde and the stars are aligned and it's Tuesday and the hacker is in position with Wireshark running on windows XP with a tethered pine phone, and we accidentally open all our ports, then we're completely vulnerable --> Ignore that guy.
Secondly, chances are that if you're building a CGI site, you won't be exposing it to the outside world at all because it's internal/personal jank that's built to do a single job, and not to look nice. If it's meant to look nice and and handle the stress of being used by a public userbase, then it won't be CGI in the first place.
(And fwiw the usecase I have does not have any authentication and doesn't need any. It's taking a POST var and doing things with it, but there are no secrets involved.)