> FTP was never integrated: it just so happened that some platforms shipped a protocol handler for it, and some browsers included an FTP protocol handler themselves. But I don’t believe you could ever, say, fetch("ftp://…").

I never tried, but i believe the relavent spec said it should work, until it was deprecated and removed from the standard https://github.com/whatwg/fetch/pull/1166

With flash - that might all be true, but there was a time when many websites required it. It might not have been a de jure standard but it was a de facto standard. To the point where a browser not supporting it was considered broken. Apple refusing to support it was incredibly controversial at the time.