Defining “secondhand smoke”
A frequent question is “why defederate from a server just because they don't block sites you don't like?” It's a valid one – it doesn't seem like a third-party block should have any effect. Usually it doesn't, but there's at least one case where it can have a significant effect.
Suppose a user on the imaginary site fairspeech.mastodon posts something “controversial” (such as “Black/trans/atheist/whatever people should be allowed to exist”). They're going to get all kinds of abusive replies from folks on foulspeech.mastodon because foulspeech can choose to pull fairspeech's posts regardless of the block.
But that's fine, right? fairspeech isn't going to display any of the replies, so unless the user goes looking, they might as well not exist.
But posit the existence of a third site: freespeech.mastodon. They're well-moderated, and would never allow any abusive replies, and they encourage their people to use personal blocks to manage abusive replies. Their users, overwhelmingly straight, white men, and proclaim themselves allies, so when the user on fairspeech seems to be getting dogpiled, they leap into the fray.
Do you see what just happened?
Suddenly that fairspeech user's mentions are full of conflict. And sure, they're not actually seeing the abusive posts, but they're seeing that they exist, and can probably guess at what they're like.
This doesn't even require bad intentions on the part of the freespeech folks. For extra credit, tell me how you'd find the difference between that and foulspeech folks using alts on freespeech to signal-boost the actual abuse.