As said in comment #1, the feature is supposed to come in future updates, but we don't know if it would be available in MIUI 12 update or not. Since this feature request is not just for Note 9 series, they'd probably implement this in whole MIUI itself, and there are chances it would be supported soon. But new features need to be tested before implementing, so it might take some time. Stay tuned.

Cheers! :-)