All credit to you for advocating for needs of marginalized groups for protected spaces to communicate, but the fediverse simply isn’t the right tool for that. It’s entire philosophy, design and implementation is centered around making everything public, from posts and comments to votes and moderation actions.

Asking the fediverse, or the activitypub protocol to allow blocking a user from responding at all is rather like asking a car to be a bike. It’s just not what it is. I can’t really concieve any way of making a decentralized public forum work like that as there is no central point that can control permissions. It might be possible to design a system where communities can control membership and posting priviledges, but even then, if it’s distributed, it would take very little for a hostile instance to simply ignore any central control and display its users posts locally, leading to the same effect as if you just mute them, leaving them visible to others, albiet only on their instance or others that cooperate with it.

I think that those who are in need of a controlled system should probably be looking at a centralized system that is run and controlled by someone, or a group, that they trust. That would give them the best chance to keep discussions private, and access to read or post controlled. Read access would need to be controlled too, or their discussions can just be mirrored to a hostile server and harassment can occur there where the poster is unaware, just as if they’d muted them.