In Firefox, go to https://test.archiveofourown.org
In the browser, choose File > Save Page As and save the page with the Format "Web page, complete"
Now open the file
The proxy notice appears. The user gets worried and contacts Support.
The proxy notice should not appear if window.location.protocol === "file:"
Followed the steps, not seeing the proxy notice! \o/
Tested three pages, no proxy notice.