This is just an example of I don't even know how many things a modern-day process will share from its parent.
By "complicated" I do not even remotely mean "unsolvable". I just mean that if you really dig down into what it means to "share nothing" in a modern operating system, it's a lot richer than it was back when fork+exec was a practical solution. There's a lot of fuzzy things that could go either way when you say "shares nothing".
I also explicitly said this wasn't unsolvable. My point isn't about technical implementations or code, my point is that the casual "I want to share nothing about the parent process" thought in sanderj's mind, and presumably a lot others, is much more ill-defined than they realize. There's a lot more state that a process has than what file descriptors are open in a modern system.
Moreover, as things like "in which container is this running" demonstrate, those are also not "create a process that has nothing to do with this process", because, again, there's a lot more to "having to do with this process" than "what file descriptors are open".
Also, as the name might have been a clue, Linux has posix_spawn: https://linux.die.net/man/3/posix_spawn. It also has a thing called "clone": https://www.man7.org/linux/man-pages/man2/clone.2.html Nor do I claim this paragraph is an entire overview of all the ways of starting a process in Linux. If you want to understand what I mean by "lots of details in a modern OS", your assignment is to carefully read the entire "clone" man page, and you'll start to see what I mean, though I'm not sure even that is all the state associated with a process nowadays.
Other operating systems either have parallel APIs to fork (e.g. the posix_spawn syscall on macOS) or do not provide fork at all (Windows).