My money is on the likelihood that most GMail users started marking these emails as spam, and GMail recognized that overriding trend and began to redirect the emails accordingly on a broader scale.

Essentially, the people FontAwesome thinks will want to hear about their new features have actually, collectively, said, "No thanks," and FontAwesome is struggling to accept that.

I think part of the problem is that Google has conflated the "mark as spam" button with "unsubscribe" and people just mash it as a shortcut to "make this email go away".