I totally agree that #Security Through Obscurity does not work, I think the key word that often gets lost is "through". Make systems as secure as you can, don't rely on them being hidden. Obscurity can actually add quite a bit. Compare a build server reachable on a public domain name to one only reachable on a tor onion service. Finding the tor onion service could take the determined attacker quite a lot of time. The key measure is time to attack vs time spent setting up defenses. 1/
I learned this lesson by operating a hidden server on a university network in a room next to a lab funded by #US three letter agencies, it was actually a feeder program, the grad students mostly went to work for those agencies. They had seen that my non-university domain name was mapped to a university IP address. They emailed me while I was on vacation, saying they were hunting for it. Two weeks later, I got back, and they still hadn't found it. They never did. That setup took me an hour.
The key thing to remember is that luck plays a role in not being discovered, in a way that it does not with proper security measures. In my example, the hunters could have gotten lucky if they just happened to think to open the right door nearby and look at the servers there.
Given that physical access to computers is a lot harder to defend against than internet access, my one hour of time vs the time they spent was quite a good payoff.