Choosing not to supply Client Subnet is entirely valid. Most services don't. But archive.is doesn't block those other services only Cloudflare. How "strange".

It's outside of my expertise. I just know that both sides have opinions and there's backers for both. Would be nice if there was an alternative to archive.is - outline?

archive.org gets my vote.

How is not supporting an optional extension "not spec compliant"?