This is more or less one of the
only part that is respected in full.
I've got to say, I've seen providers without proper discovery, I've seen providers with strange hybrid flows and I'm pretty sure most providers don't implement the request/response signing and encryption (after all, if you want insanity, you'll be more than happy with SAML). But I've never seen a provider who doesn't implement an ID token.
I think everybody would agree that OpenID Connect without an ID token is just plain OAuth 2.