I tested RPM notch filter in Betaflight 4.0. It is awesome.

At the moment of publishing this video only some F4 and F7 flight controllers support RPM notch filter. A BLHeli32 ESC is also required, and it needs to be flashed with experimental firmware that enables bidirectional DShot support. And of course you need the latest development build of Betaflight 4.0. (Pretty scarce) instructions in Betaflight Wiki: Great explanation and setup instructions on UAV Tech channel (shout-out to Ma