I'm still unsure, but I couldn't obtain that info so far. Yes, I know, stage manager will only work on M1, but does the iPad mini now makes use of the whole screen when connected to an external display or do we still have 4:3?
Still mirrored with black bars. The mini 5 is 4:3. The mini 6 has a bit wider aspect ratio which I believe is somewhere around 3:2 (so smaller black bars).
I can confirm in the iPad Mini 6th Gen that the external monitor support does not have the new features for external monitors. As @rui no onna said, there are black bars and the aspect ratio is around 3:2.