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.

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.