Update: Gary Illyes from Google now officially confirmed the update on Twitter, more details at the end of this story.
Gary said in his response to questions about this Fred update that when they discussed the industry chatter on this update, his "search leads" (I guess his bosses) decided not to discuss it with the public. Then Gary said he will say one thing, that the sites impacted by this update should have been because what they are doing are specifically against Google's webmaster guidelines. Gary said the answer to why sites got hit by this is in the existing webmaster guidelines.
I did try to get Gary to give us one specific guideline that these sites violated but he wouldn't add anything more about this update.
Again, if I had to guess based on reviewing over 100 sites that saw significant drops on March 8th, I'd said most had low value content with the goal of revenue generation only.
Here are all the tweets I could find covering this from the session, which I sat in and heard myself:
Google talked to the leads up Fred, and we decided to not talk about this update. said methode #smx— Ben Schmaderer (@muc_webdesigner) March 23, 2017
What methode can say about Fred, your answer is in our webmaster guidelines. #smx— Ben Schmaderer (@muc_webdesigner) March 23, 2017
Sounds like a confirmation to me.
Update: Gary tweeted after this story that "obviously there was an update. Why would we deny that."
@rustybrick will. Obviously there was an update. Why would we deny that— Gary Illyes ᕕ( ᐛ )ᕗ (@methode) March 24, 2017
Forum discussion at Twitter.