Flagged this two years ago (yikes).
Any indexing state can move into the 'crawled - currently not indexed'.
Because it hasn't been recrawled for a while.
These pages will then move to 'Discovered - currently not indexed' and finally to 'URL is unknown to Google'.
Index Coverage update: I have noticed this a lot over the past several months. Sites seeing categories of "Not indexed" urls drop heavily, which look like they were routed to "Crawled, not indexed". So it's like the categories flipped...
I have seen that across a number of sites I have audited recently. For example, see 'alt page with proper canonical' drop, 'redirects' drop, etc., and CNI surges like mad at the exact same time. Just an interesting observation. :)