Look, I the English language (as in many) often, words are left out, but the MEANING is still very much there... for the native speaker. (For others, I can only apologize.)
So what has been left out, here?
In MY book, first on all, a comma has been left out:
"This page, intentionally left blank..." but then something else would be missing. I believe we call that a non defining clause. It would , then, express a thought, "This page, which was intentionally left blank..."
But it isn't really a non-defining clause, either. And no, it is NOT correct English as it was written, at least it is not a proper sentence for the very reason you express. "no verb!" (of any kind). It's sort of like "No Smoking". It's information that everyone has come to understand though it isn't (technically) correctly expressed.
The sentence (if it, in fact IS one) should read, "This page was intentionally left blank." or even "This page has been left blank intentionally."
We are left to figure out the meaning. We cannot fix the grammar. "Men at work" ; "No Dogs"; "Keep Right".
Many instructional, or information notices lack proper or complete grammar constructional components. Sad but true! Easy come, easy go! Not to worry!