@atomicpoet @sambook Its technically possible now, but I think it'll get really interesting once Mastodon gets Markdown into its public release.
You could currently follow a Substack writer via email, and automate that the email gets reuploaded on wordpress. Enable Activitpub on the WordPress blog, and now you can follow the blog in Mastodon.
I definitely agree that the substack owners dont have an interest in making this happen, but thats the fun part of open platforms, we can just do it