We know that all new websites launched after July 1, 2019 use mobile-first indexing in the Google index by default. But what if you started a brand new website with an existing old domain name that hasn't switched to mobile-first indexing?
Google's John Mueller said no, it wouldn't use mobile-first indexing by default. Google needs to access the new website when it detects it and do this rating to move it or not like it did with websites launched before July 1, 2019.
John said on Twitter that on a simplistic level, Google "keeps a list of domains that we think are not ready yet". If this old domain is on this list, Google will not consider a relaunch of this domain as a new domain. Of course, the new site will eventually be detected and crawled and automatically determined whether the site is ready for mobile-first indexing. However, it is not just removed from the "Websites not ready for mobile-first indexing" list because a new website has started on it.
John said, "If you start a new site on an existing domain, the status of the domain doesn't change automatically. We'll continue to check that the domain is overall ready before removing it from this list."
Here are the tweets:
Basically, we keep a list of domains that we think are not yet ready. When you start a new site in an existing domain, the status of the domain does not change automatically. We'll continue to check that the domain is overall ready before removing it from this list.
– 🍌 John 🍌 (@JohnMu) August 18, 2020
This is one of the "good to know" things that doesn't make much practical difference.
Forum discussion on Twitter.