Adding a brand new anonymous comment method is a potential conduit for spam. Since it's still useful functionality, we're now hiding it behind a filter to allow plugins and themes to turn it on if they do want it.
Props helen, rachelbaker, joehoyle.
Fixes#38855.
Built from https://develop.svn.wordpress.org/trunk@39327
git-svn-id: http://core.svn.wordpress.org/trunk@39267 1a063a9b-81f0-0310-95a4-ce76da25c4cd