Gary Illyes from Google confirmed that a vast amount of URLs were dedindexed from the Google Search index in February. This was due to a change in Google's perception of the site. The statement was made at the SERP Conf in mid-April.
Illyes explained that the number of URLs crawled but not indexed, which can be seen in Search Console, could indicate general quality issues with a site. He noted that a high number of these URLs could be a sign that Google's perception of the site has changed.
However, he clarified that a spike in "crawled but not indexed" notices is not necessarily because Google considers the site low quality. It is more likely due to a technical issue with the site.
In February, there were several unconfirmed Google search ranking updates. Many SEOs reported their pages dropping out of the Google Search index.
Other topics covered by Illyes at the event included Google's need for very few links to rank pages, actions taken on some Parasite SEO in a recent update, and the decision not to change the 301 redirect signals for ranking and SEO.
