codesections@write_as is trying be be one (though I've seen a number of people advising waiting until the release their source code; they've promised to become open source but aren't right now)
It's also very easy to self-host a blog with a static site generator. I love #gutenberg, but people also like #hugo or #Jekyll. And #Netlify provides good, free hosting for static sites. (#gitHub pages too)
All Mike Gerwitz's GNU Social Instance content and data are available under the