It's tough to say as the message is pretty vague. They don't really mention what you violated.
There are lots of people (non datafeedr customers) in the same boat:
I don't see how adding the shortcode to your posts would cause a decline as long as the post content was unique.
I do see that spammers have started commenting on your blog posts.
You seem to have spam comments and their respective spam links on most of your articles. It might be all of the spam comments that raised a flag with Google....
I would first remove all of those spammer's comments from your site and moderate all comments.
Then, I would look at the this link
and try to determine if your site has similarities to other's sites that got removed.