There exist third party, so they are allowed - but not ordinary VSTs. I think Steinberg use VST MA style.
Thereby my request for a wrapper to insert VST ones, like Piz midi plugins.(still on first page I think).
Not much worth having - but Frank’s midi plugins have install for Cubase midi inserts:
I thinks it’s seven years since I bought them, and nothing new as I recall.
So don’t expect new things to happend.
I talked to Waves some years ago - if they were to make midi plugins - but they had no such plans.