It's already starting to fail on some of the major sites. Recently it's stopped working on the Wall Street Journal, the New York Times, and a couple of others that I can't remember offhand.
Interestingly, the blocking seems to be on the Outline end, because the site says "We're sorry, but this URL is not supported by Outline" if you try to use it on an article from one of those sites (example: https://outline.com/WytaqY). The message when scraping a page fails is "We're sorry. This page failed to Outline."
So it's not that the sites themselves have figured out how to block it, but they seem to have gotten Outline to specifically exclude them.
