PeerJ has released "Peerj Preprints to Stop Accepting New Preprints Sep 30th 2019."
Here's an excerpt:
We started the PeerJ organization primarily to provide a superior peer-reviewed experience shaped by its Academic Editors. A secondary goal was to bring preprints back to biology. As the community's appetite for preprints has now been cemented, we too want to focus our efforts more fully on our portfolio of peer-reviewed journals and primary mission. While PeerJ Preprints has been successful and pioneering, the academic community is now well-served with other preprint venue options (and new ones are continuously being created to fill necessary areas, many of which are not tied so closely to a specific publication).