dig +short -t txt ubuntu.com|grep goog
    "google-site-verification=8CcvY6fsgRbiFk2nyEaiA9rE-ykXOcWUPjJKlO6P4vk"

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.