I agree with your general point on the increasing lack of discoverability, but this feature, in fairness to Apple, is "hidden" behind basically the only graphical icon in the Messages app, on the main screen, of a pen on paper, practically screaming to be touched, and once touched, jumps to an email-composer-esq screen with a "to:" text box with the cursor flashing in it.

I have to say, I am a bit surprised I never touched it. It truly is the only icon there, I dunno what I could have thought it meant, but I guess it never occurred to me that it would bring up contacts.