I think Substack reached out to a lot of the people who were getting a lot of traffic to "encourage " a subscription model. Midwestern Doctor wrote about it at the first anniversary of starting his stack. (MWD chose not to make the pay wall, but simply said, if you want to give, awesome, thanks.) But since then, I have noticed more pay walls.

Alex Berenson I believe does a variation where the articles are pay walled first and released to all a few days later.