On Mastodon, Followers-only posts are only visible to your followers -- and to admins of any instances your followers on. But if you haven't turned on "approve followes", anybody who's logged in to an instance you haven't blocked can follow you and get access to your followers-only posts.

In your view, are followers-only posts public?

The linked post is a Mastodon poll, and I'll also put in replies here so that you can just upvote the ones you agree with!