Google Organic Search Metrics
Google Search Console (GSC)/ webmaster tools provides insights into the way users see your organic Google SERPs. Unfortunately, the dashboard data only goes back 90 days, so when Google makes a change, it is difficult to normalize the data.
So, here is what changed:
For some websites, Google displayed site links under the primary listing. So, one search with one listing provided multiple Landing Page impressions in the GSC. Google removed this late last year. So, sites for December and January that had a lot of impressions due to site links, suddenly say their impressions drop. In some cases drastically.
The good news is that Google dropped organic site links because they were getting minimal clicks. So the actual site traffic should not have been measurably impacted. But if you keep track of up stream metrics, this throws a wrench in your data.
(google also delivers multiple listings for a site in one search, legitimately creating multiple landing page impressions for one query).