> What if websites start requiring specific files to exist before allowing access?

This is certainly going to be immediately abused for encrypted storage of persistent cookies and tracking identifiers.

- If my site generates cat pics, I'll put identifiers in the metadata fields of the image format.

- If my site generates markdown, I'll put identifiers in an alternate data stream (Windows) or encoded in the whitespace.

Since the files exist outside of the sandbox, they'll be outside of the scope of privacy features like clearing the cache or cookies, and outside of the reach of adblocker extensions.