TLDR: effective of Jan 1st, 2018 online platforms are required to delete posts without warrant under certain provisions. The law passed during German federal elections to counter "fake news" and "hate speech" (and also to counter foreign nations messing with the elections), even with obvious questions regarding its legality under constitutional and fundamental rights. Tomorrow it's going to be officially challenged before a lower court as a first step to take it to the German constitutional court ("Supreme Court").
Already discussed in https://news.ycombinator.com/item?id=17108353 where this law was portrayed in a positive light by NYT, though