Is there a Substack API for publications, posts and Notes?

Yes. Social Fetch has 26 Substack routes under /v1/substack covering publications, archives, post bodies, sitemaps, in-publication search, recommendations, feeds, posts, comments, reactors, restackers, reader profiles, Notes, subscriptions, search, categories, the explore feed and the publication directory. Authenticate with your Social Fetch API key in x-api-key.