I have yet to see any webmaster say they (a) got a notification about their site being enabled for the mobile first indexing and (b) them seeing their cache or other signals that their site has moved over.
Google will not be shifting all websites to the new mobile-first indexing immediately but only the first batch. This is the first time Google is moving various sites to this mobile-first indexing process which uses the pages' phone-optimized versions for indexing and ranking. In the following years, Google started giving a search ranking boost to sites with mobile versions, but only in Google Search Mobile, leaving its Desktop Search unaltered.
Don't call it the mobile-first index, but rather call it mobile-first indexing. The "More" tab will load more information on the same page where users were earlier able to see the Next Tab which took them directly to the next page instead of showing more information on the front page. But now, with mobile-first indexing, the mobile version of the page will be used by Google for indexing and ranking and this will benefit the mobile searchers. So it doesn't come as a surprise that Google wants to acquire it to better their search results. Historically, the desktop version was indexed, but increasingly, we will be using the mobile versions of content.
"If you only have desktop content, you will continue to be represented in our index", Google announced. This is known as content parity, and it's what Google will be looking for when it crawls your site in the near future.
However this appears to be a test as not all users are seeing the changes, meaning that there is no guarantee that Google will implement this new feature, but what say you?
By "primarily mobile", Google is referring to the fact that the majority of people who use Google search today now do so from mobile devices, and have done so since 2015.
Inbound marketers have been following the rise of mobile marketing for years, but considering how ingrained mobile devices are in our lives as both consumers and professionals today, it shouldn't really be considered a "trend".