@stefan I have quite a bunch of ideas for moderation that could prevent harassment in the first place, tbh, but chances of Masto devs ever implementing anything like it are about minus 9000%
What can be implemented re: reply controls is, basically, selective muting. A post could indicate "only people XY may reply" (i.e.: followers), fellow vanilla Mastodon servers would respect that, other ActivityPub software may or may not respect that, and bad actors certainly wouldn't. So while it may hide unwanted replies from cooperating parties, it would only ever do so on a good faith basis.
Twitter could do reply controls because Twitter is one company. All user accounts, all posts, all are owned by Twitter. It rules absolutely, for better or worse. That isn't possible with ActivityPub, where each post, each like, each follow, is just servers sending "hey, I did this thing" announcements into the ether and other servers deciding how to respond.