We first started working on mobile-first indexing a number of years ago. Since then, we
switched to indexing the majority of web pages with Googlebot-Smartphone. We’ve now
determined that some sites are still not ready to be shifted over due to various,
unexpected challenges. Because of this, we’re now leaving the timeline
open for the last steps of mobile-first indexing.
Source: Google Webmaster Central Blog
Link: A 2021 update on mobile indexing
