The one that annoys me most is the accept-language header which is almost entirely ignored in favour of GeoIP lookups to figure out regionality... which I find super odd; as if people are walking around using a browser in a language they don't speak. (or, an operating system configured for a language they don't speak).
ETAG's though, are a bit fraught- if you're a company, a security scan will fire if an etag is detected because you might be able to figure out the inode on the filesystem based on it... which, idk why that's a security problem eitherway[0], but it's common for there to be false-positives[1]... which makes people not respect the header.
Last-Modified should work though, I love the idea of checking headers and not content.
I think people don't care to imagine the computer doing as little as possible to get the job done, and instead use the near unlimited computing power to just avoid thinking about consequences.
[0]: https://www.pentestpartners.com/security-blog/vulnerabilitie...