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

After updating to big sur, I can no longer mount the root as writable (even with SIP disabled):

sudo mount -uw /
mount_apfs: volume could not be mounted: Permission denied
mount: / failed with 66

How can I fix this?

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

My company requires to read/write environment informations under /data/webapps/appenv. Therefore, this workaround shared by a colleague of mine might not answer this question directly, but it is super useful if you want to write something under your root path!

1. Create a file: sudo vim /etc/synthetic.conf.

2. Write content: <dir>⇥/System/Volumes/Data/<dir>. Notice: the ⇥ is a tab!

   In my case, it looks like this: data⇥/System/Volumes/Data/data

3. Go to /System/Volumes/Data and do sudo mkdir <dir>.

4. Then do sudo chmod -R 777 <dir>.

5. Finally, reboot, and your directory <dir> should now be accessible.