Most big media sites now have "you read X free articles this month, you need a subscription to continue" popups. Medium doesn't require you to sign up to continue, but it acts and reads just like those in its cutesy way, hoping the user infers the requirement to sign up.

That popup is a UX dark pattern, and for me puts them in the same category as LinkedIn with its various means of lulling you into actions you wouldn't consciously take.