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As expected, the instance block option was implimented poorly. It fails to block posts and comments from users on the blocked instance, if the post or comment is made on an instance that is both federated and not blocked.

But it will not show replies from the user on the blocked instance in your inbox.

I don't see how rushing a vote would have been better.

This feels like an uninformed take. Have you interacted with people on Mastodon via a Lemmy instance? They're clueless about the fact that they are replying to a forum post or that there's more than one thread of the conversation going on that most people can see and interact with. The conversations tend to be awkward.

The size of the userbase being merged in seems more important than the commercial status of the service that's federating.

Users can now individually block instances in Lemmy 0.19,

Assuming that the Lemmy devs implemented this thoughtfully, thoroughly, and correctly on their fist attempt is a bad bet. Even their description seems to indicate that users would be able to see comments and direct messages from blocked instances.

My bigger concern is content being overwhelmed by a flood of accounts that have a different online culture from what has grown and is still developing here.

For those out of the loop, Mark Zuckerberg recently made an announcement on Threads

This is part of the ActivityPub protocol, but I haven't looked into it enough to know how it's defined.