It's part of the VST SDK. I think it's called VST Module Components. There is very little info out there about it and limited documentation. You have to put it (a .dll) in the folder in ProgramFiles/Cubase/Components.I would love to know your authoring process to get MIDI inserts in Cubase. My impression was that the inserts available could not be changed, but that is mainly based on a lack of information on adding MIDI plugins. Exciting!
It has a few differences to a VST and even the MIDI standard which took me a while to figure out. Like instead of sending NoteOff messages you set the length of a note on the NoteOn. It processes MIDI in blocks just like a normal VST.
It only works in Cubase and that is why it isn't popular.
I have a feeling that the NoteFX in Studio One may also use this but there is no way to add any. I've even tried just dropping the FX in different Studio One folders but it doesn't change anything.