Unable to mount read-write in Big Sur 11.2.2 in Single User Mode

I'm on Big Sur 11.2.2, Single User Mode. When I use /sbin/mount -uw /, it says mount_apfs: volume could not be mounted: Permission denied and failed with 66.

I have read this post. But mount -uw /System/Volumes/Data gives mount: unknown special file or file system /System/Volumes/Data/. Also, csrutil enable --without fs in recovery mode changes nothing. Errors still occurs. (Previous tries are based on SIP entirely disabled.)

The commands mount -P 1; /usr/libexec/init_data_protection; mount -P 2 in the prompt during startup works. However, I don't know what it's mounting.

PS: current SIP status:

Configuration:
    Apple Internal: disabled
    Kext Signing: enabled
    Filesystem Protections: disabled
    Debugging Restrictions: enabled
    DTrace Restrictions: enabled
    NVRAM Protections: enabled
    BaseSystem Verification: enabled

This is an unsupported configuration, likely to break in the future and leave your machine in an unknown state.

Solution 1:

The limitations of Single-User Mode and the changes in Mojave & Catalina have been covered before. In particular why you can't unmount the system volume. I don't think it needs repeating again (unless someone want to add changes in Big Sur). Read this:

fsck_apfs not working from terminal in single user mode