If I google `"libxmlb2" site:packages.ubuntu.com`, google returns no results. I've tried this with a couple of other package names, like cups and firefox. The robots.txt is permissive. The Debian package repo, packages.debian.org is showing up and duckduckgo has the ubuntu package repo indexed.
I would guess that someone may have used their search console interface to purge/disallow everything despite the robots.txt allowing it. Another possibility is that after purging the caches they may have blocked google. Only the admin that verified the ubuntu.com site in the Google search console would know for sure.
If I google `"libxmlb2" site:packages.ubuntu.com`, google returns no results. I've tried this with a couple of other package names, like cups and firefox. The robots.txt is permissive. The Debian package repo, packages.debian.org is showing up and duckduckgo has the ubuntu package repo indexed.