From the README:

Mastodon 4.x radically changed the frontend, to much dismay from the actual community. It’s now a (slow) webapp, which requires access to lots of API routes that were previously unavailable to unauthenticated parties. It gives the public a much deeper view into your (private) community, both non-techincal (instance home pages now show an ‘explore’ page nobody asked for, that shows public content from instances you federate with. [you have to fully disable trending]), and on a technical level (toots and search API are publicly available allowing for much easier programmatic scraping).