Alright, we appear to have uncovered something with the help of y'all.
Twitter had a featured apparently called "Quality Filter" - this was probably turned on at some point unbeknownst to me once I crossed X-number of followers but was opt-in otherwise.
This was undoubtedly the thing that hid nasty replies and prevented them from showing up in my notifs. And if this wasn't on for you (or you didn't have access) that may explain why my lived experience Over There has been easier than here.
Hm~ what's your benchmark for what moderation should care about? It seems that clearly you (and others) would benefit from having way stricter moderation, so it sounds like there's raison d'etre of an instance that has such stricter moderation (not only on its own users, but also on remote users by way of instance-wide mutes and blocks targeting individual remote users). (It might be impractical to actually run such an instance due to having to police ~all of fedi, but that sounds like something where "moderation subscriptions" might actually work.)