They seem to have that warning on every single page of catalog.archives.gov. That solution seems much easier than figuring out whether each individual item requires a warning or not.

OP is onto something - the mesage should reflect that in such case.

One thing I learned at work preparing countless PPTs and bug reports is that people will misunderstand or overinterpret whatever you write, so better write precisely in an idiot-proof manner and provide context.

I personally would move this to be the navy-colored navigation bar. It could be reasonably understood that this section does not directly represent the item shown at the page. Similar stuff should be together, after all - and that bar links to e.g. accessibility statement, which is also a kind of meta-content.