I got an answer from Support. If you published the story before Medium had email following, you can send it now. They started email subs in December of 2020.

https://blog.medium.com/level-up-your-following-b944be6b3cbe

I asked if they’d ever thought about letting us send to people who signed up AFTER the story was published. Duh, no, of course they never considered doing that. They said they’d run it up the flagpole.