[remram]

That sounds like a weird way to implement a built-in command (the IPC handler still needs to be built-in, so you didn't really remove it from the shell). And then if course that binary would be available globally but only work if you're using the shell it's built for...

[daptaq]

Yeah, that is why it isn't done. I am just pointing out the pointless fact that you could write a cd command that wasn't itself built-in.

[remram]

You could hook up to the parent process with ptrace or the equivalent debugging API of your OS of choice and forcibly change the directory. Stackoverflow even has an implementation: https://unix.stackexchange.com/a/114262/81005