For the Debian situation specifically:
> It would've been nice if there was a quick serverless solution where you could just say a URL to look for newer versions of a single package, and both apt and dnf would remember that and update a package.
Have you considered just hosting your own Debian repo? It's basically what you're looking for here. You don't need more infrastructure, either, you can host them straight in github pages with aptly or reprepro or something, and drop new releases in there from CI.