Apple - Can I mount the root (system) filesystem as writable in Big Sur?

DISCLAIMER: have a good backup and plan to erase and reinstall your Mac from the internet before trying this procedure. A mistake can prevent your system from starting without assistance.

1. Disable FileVault
2. Reboot into recovery mode and run:

   csrutil authenticated-root disable
   

3. Reboot back into MacOS
4. Find your root mount's device - run mount and chop off the last s, e.g. if your root is /dev/disk1s2s3, you'll mount /dev/disk1s2
5. Create a new directory, for example ~/mount

   mkdir -p -m777 ~/mount
   

6. Run sudo mount -o nobrowse -t apfs DISK_PATH MOUNT_PATH, using the values from above

   sudo mount -o nobrowse -t apfs /dev/disk1s2 ~/mount
   

7. Modify the files under the mounted directory
8. Run sudo bless --folder MOUNT_PATH/System/Library/CoreServices --bootefi --create-snapshot
9. Reboot your system, and the changes will take place