How to disable dm-verity on Android with "user" build type ROM?

I have One Plus 6T Android device which has a ROM of build type user. This device is rooted with Magisk and I want to disable dm-verity on this device. I have tried to run adb disable-verity but I got an error saying verity cannot be disabled/enabled - USER build Is there any way to disable this? Goal: Want to place customized sepolicy file under system_root directory. I have tried to do it with following commands:

C:\Users\GPU-Test>adb shell
OnePlus6T:/ $ su
OnePlus6T:/ # mount -o rw,remount /system_root
OnePlus6T:/ # cp /sdcard/selinux_policy_modified /system_root/sepolicy
cp: /system_root/sepolicy: Permission denied
1|OnePlus6T:/ # cp -f /sdcard/selinux_policy_modified /system_root/sepolicy
cp: /system_root/sepolicy: No such file or directory
1|OnePlus6T:/ # ^C
130|OnePlus6T:/ # exit
130|OnePlus6T:/ $ exit

C:\Users\GPU-Test>adb disable-verity
verity cannot be disabled/enabled - USER build

Also tried to disable Preserve force encryption and Preserve AVB 2.0/dm-verity checkbox but still the above commands have same result. Also on reboot these check-boxes enables again automatically.