If this is new, it is perhaps an attempt by Medium to prevent the RSS spamming on Facebook that has gotten the entire Medium site pretty much banned from Facebook link sharing.

No individual MPP writer could, legitimately, write and publish enough (outside of being an Editor for a very large publication) to hit such limits. They could, in theory, if they were scraping and dumping plagiarized content or all their old blogs and the dated content therein. In which case, I applaud Medium's efforts to thwart them.