People don’t want to see uninteresting replies and casts.

Best way to solve for that is weight historical inbound engagement for a user, i.e. PageRank-like score for every user.

Disproportionally hurts new accounts. Making it harder to get started.

What if instead of doing that automatically, you implement a "interaction quality rating" system? You let each user rate an interaction on their cast, from a pov of "was it relevant/useful or not?" Over time, spammy and low quality accounts will move to the shadow ban box naturally.