It's common to heavily abbreviate signs and notes.
This can allow some amusing misreadings. E.g:
Stop the police from doing what?
Stop Immigration Control
Yes, I too agree with more open borders.
It's a bit mean to comment on their learning problems in a road sign.
But such misreadings are deliberate humour - it's clear what they really mean.
And while some people do argue that such notes and signs are ungrammatical, truly they're using a different grammar than full sentences. We don't even notice that a shop wouldn't be using full sentences in having a sign saying "Books" or "Fish & Chips" rather than "This premises is a shop that sells books" and "This premises is a shop that sells fish & chips".
Indeed, which seems the stranger:
No milk please
We do not require any milk today, please.
The former is a common enough notice to see near a door in places that have daily milk deliveries, the latter seems fussy at best.
And so it is here. The appropriate sentence would be either "This page is intentionally left blank." or "This page is to be intentionally left blank." (Depending on whether in making it a full sentence we make it a description or an instruction; as a note it serves as both). As a note though, the omission is perfectly normal.
The even more concise "Blank Page" is also commonly found.