Is it possible to use UITextView in #TextKit2 with a subclass of NSTextContentManager without using NSTextStorage as backing store?

It is not. UITextView requires UITextStorage as a backing (and provides access to it via its textStorage property).

Thanks for confirmation.

So basically I have to subclass UIView and implements UITextInput to start with? On Mac, this may mean subclassing NSView and implements NSTextInput / NSTextInputClient?

I won't comment on AppKit (I don't have the background to be reliable), but for UIKit, yes, that should be possible.