Ask HN: When did medium.com introduce a soft paywall?

I just followed this link from the HN front page: https://medium.com/@melissamcewen/who-killed-the-junior-deve...

I was greeted with a popup informing me that I had now read one of three free stories.

It was introduced last August.

Most posts are not paywalled, because users need to apply to a partner program and make sure their posts adhere to particular criteria in order to have the option of putting their posts behind the paywall.