Google is sending out notifications for mobile-first indexing issues detected on your site. And in that notification it says "Google expects to apply mobile-first indexing to all websites in the next six to twelve months." Yes, probably by the end of this year, all indexing will be done using mobile-first indexing at Google.
As I covered last night at Search Engine Land, Kyle Sutton shared a screen shot with me on Twitter of this new notification. We knew Google would begin notifying site owners via Search Console about mobile-first indexing issues. But I didn't know Google would put a timeline on it.
The timeline is 6-12 months, so probably by the end of this year.
Here is a screen shot of the notification:
So I would take these notifications seriously if you get one. You have less than a year to make sure your site is ready for mobile-first indexing, if it has not yet been switched over.
Forum discussion at Twitter.