Services that support WebSub have specific requirements. The link elements or the HTML headers are what they require. I think the best solution might be to use inline namespaces like we talked about and just put them at the bottom of the channel contents (after the items).
We still need the URL namespace with Atom in it, but at least it's not at the top of the feed. We might lose some parsers that are just looking for atom:link elements and not doing proper namespace resolution, but I think it's the best middle ground. Maybe ask the braintrust?
<link xmlns="http://www.w3.org/2005/Atom" rel="hub" href="https://rpc.rsscloud.io/websub"/>
<link xmlns="http://www.w3.org/2005/Atom" rel="self" type="application/rss+xml" href="https://feed.example/rss"/>