To allow people to review their followers before accepting them ("follow requests") all subscriptions in ActivityPub require the followed actor to send back an Accept activity. This, and the Webfinger requirement to confirm the username@domain address that predates ActivityPub, preclude plain static sites from being followable actors as seen from Mastodon. Although nothing prevents you from publishing a static feed in the ActivityStreams format, this would require a different, feed-like subscription mechanism where the publisher is not aware of subscribers and the subscriber has to poll the publisher periodically.