I think you’re confused.
There are reasons to defederate. Malicious instances are one example.
There are many reasons not to deferederate, like users on a particular instance being assholes to a specific user or group of users.
A user cannot, today, block users from an instance. They can block individual users, they can block communities, but individual blocks can be easily worked around.
A better option is for that user to have the option to block an instance (and its users) that others are fine with interacting with.
You are trying to rework that into something its not.
Edit: Ahh, the no reply just downvote. Says everything. Goodbye.
Was that the file transfer allowed for remote code execution one? That’d be the one that sticks out to me. 3 or 4 years ago iirc?
Edit: CVE-2021-27649 is the one that came to mind, not sure if that’s the one you’re referring to.