I had completely forgotten to mention that allowing a user to narrow the scope of replies seems to enable a particular form of harassment, predominantly employed for racist abuse of black users.
Described here:
https://gts.sadauskas.id.au/@aj/statuses/01K6Z6ZAXY1SB46X6PAFDKM9S9
And in a thread I wrote earlier:
https://union.place/@feralthoughts/115345717793512758
(continues)