Interesting. I've been using Unix systems for 30 years and never noticed this.

On my Fedora system, /usr/bin/cd is just a shell script that invokes the shell builtin:

  #!/usr/bin/sh
  builtin cd "$@"

I suppose it could be useful for testing whether a directory exists with search permissions for the current user safely in a multithreaded program that relies on the current directory remaining constant.

Yeah, it's typically a shell built-in since you'd want cd to change the cwd for the shell process itself. Child processes (like commands being executed in the shell) can inherit the parent shell's cwd but AFAIK the opposite isn't true.

Wait, how did the `cd` executable used to work in old Unix? Did it instruct the kernel to reassign the CWD of the parent process?