Creating a New Folder in Macintosh HD

You cannot modify the top level of the startup drive. Save your files in your home folder.

Pretty much the security model changed. Both Catalina and Big Sur mount The OS is mounted read only so nothing can attack it.

further, Big Sur split it into an OS volume and a data volume. The OS is mounted from a sealed snapshot made from the OS volume.

What has changed in Big Sur and Catalina that makes this impossible? I can do it in Mojave and below. Feels like an arbitrary step backwards.