I had occasion to revise a site in which I had to manually insert a declaration on every page prior to the require() for the header. Then I had to double-check every page to make sure I hadn't missed any (which I had) before uploading it.
It occurred to me that it wouldn't be that hard to code a finder that would identify pages that do not contain the search string. I don't think I've seen this in any editor I've used, but it would be an occasionally useful feature.
Richard