Since September 14, Google has been sending out notices around mobile-first indexing being enabled for websites to a large number of webmasters. It appears that this batch of notices is by far the single largest batch of notices seen since they officially began sending these types of batch notifications around the mobile-first indexing change.
So, if you did get notices like this, there shouldn’t be any reason to worry, as you probably aren’t the only one to get these notices.
Google explained their “crawling, indexing and ranking systems have typically used the desktop version of a page’s content, which may cause issues for mobile searchers when that version is vastly different from the mobile version. Mobile-first indexing means that we’ll use the mobile version of the page for indexing and ranking, to better help our — primarily mobile — users find what they’re looking for.” In short, Google is crawling the web as a smartphone browser would render your website, as opposed to how a desktop browser would.
It’s Google’s hope that this isn’t going to have an impact on your rankings. But if the content on your site is vastly different on desktop versus mobile, then it’s possible it can impact your rankings on Google search.
Google issued these clarifications a few months back with new and clearer advice around this change.