Apple controls the app store. I think during MacOS 11 we were about version 2.3 or so (the first version to run on the M series Macs), not MD Pro X.
We have some reports of older devices showing the appropriate older version to load, but this is not under our control.
One user recently report success loading 2.3 on an older device with a Pro X subscription - see
https://mididesigner.com/qa/8099
Search for midi designer on your Mac App Store. Select the iPad/iPhone tab. If MIDI Designer is shown, then it can be loaded.
Let’s see if a user still on macOS 11 has any more insight. And they may have loaded the app while macOS 11 support was current.
I have copied this question to the facebook group.