MIXER.MIX still usable with Plane 4.4.x?

I recently upgraded a Cube Black from Plane-3.8 to Plane-4.4. It was used with a custom mixer that was stored as MIXER.MIX on the SD card. As far as I know, this mixer file was part of the PX4 framework. Is it still usable in Plane 4.4 and higher?

And if not, how can the mixer be replaced?