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I am not sure if this is a bug or not but Dr. Pete Meyers from Moz reported on his Mozcast features tool that in-depth articles are completely gone as of this weekend.
Pete verified the data manually and said there is no sign of them at all.
Verified that I can't find any current SERPs with In-depth Articles that previously had them. Both visual and code markers gone.
— Dr. Pete Meyers (@dr_pete) April 9, 2016
I personally checked dozens of examples and found no in-depth articles come up for me.
Google launched them in 2013 and has changed them overtime, in terms of how they look in the search results. It is a bit surprising that Google removed them, it might be a bug. I will reach out to Google to find out.
Here is a screen shot of Mozcast:
Forum discussion at Twitter.
