Sebastian Lasse on Nostr: well, it is a general problem: If the biggest software does it, each other follows. ...

well, it is a general problem:
If the biggest software does it, each other follows.

Anyway, trying to puzzle the pieces.
The Conformance Section https://www.w3.org/TR/activitypub/#conformance says:
You can implement either
• Client to Server
• Server to Server
• or both
Mastodon does the second bullet ;)
The Activity “Block” is specified as as C2S action as https://www.w3.org/TR/activitypub/#block-activity-outbox

My fedi-software does both (fully conformant). So, it follows above specification and so includes https://www.w3.org/TR/activitypub/#delete-activity-outbox replies.
However, this might be a bit underspecified but it is useless if any software rolls out their own Client-to-Server-API.

So, I'd say it is “rather a software issue”.

Anyway. What I could not find in the spec. at all was:
What would happen to replies if an object gets deleted. This is what I am interpreting as the replies should stay.
I can only say that this is also a legal issue, it is e.g. my only way to show how I reacted to deleted posts of Nazis.
Opinions?