It seems that Google’s mobile-first indexing deadline has become open-ended. According to John Mueller of Google, the company decided that they will leave the “the timeline [open] for the last steps of mobile-first indexing”. This comes from the company blog.
The blog stated:
“Earlier, we thought that we could complete the move to mobile-first indexing by March 2021. However, we found that some sites were facing unexpectedly difficult challenges and we wanted to accommodate their timelines. We currently don’t have a specific final date for the move to mobile-first indexing and want to be thoughtful about the remaining bigger steps in that direction.”
The deadline was originally postponed from September 2020 to March 2021, but it came and went. Now, it seems the deadline really isn’t much of a deadline anymore.