That's not true at all. You can use recovery mode to trivially revert back to the OS that was installed when the computer was purchased. If that's pre-Mojave then you can just upgrade back to Mojave afterwards.
What if the Mac had a newer OS than Mojave originally installed on it? That is how I interpreted the parent poster's comment. Given this interpretation, I'm don't think I'd have the expectation to be able to install an earlier OS.
With that interpretation, you'd be correct but I don't think you've ever been able to downgrade to something earlier than what it came with since the older OS wouldn't include the appropriate drivers or kexts to properly run the hardware.
