It's because of the email sent a few days ago by Medium Writer Support

"A look back at January, more Partner Program updates, and academic office hours"

Extract:

A change is coming to the way we approach stories in the Partner Program about Medium, what we often call meta stories. Those kinds of stories are already something we try to keep out of your feeds as a reader unless you've specifically asked to follow them. The goal of the Medium Partner Program is to deliver value to readers and writers, and we often hear from our members that these kinds of stories aren't the ones they want to read, much less pay for. You're free to write meta stories, we just don't want Partner Program funds going to them. Payments seem to incentivize extra navel gazing and unwanted get-rich-quick culture. In March, we will start notifying writers when we see their meta stories behind the paywall, and then begin removing them.