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julianFor example, let's say I link out to Evan's profile here. If NodeBB knew that this link had an alternative AP endpoint, then we could redirect the user instead to the local representation of his profile
Wouldn't Evan's AP endpoint be the same as his HTML endpoint?
Most platforms are going to send you to the authoritative profile, which is the one at the user's server.
And if you wanted to redirect a link to a local profile instead of his official profile, you don't need an endpoint to do that. You could simply parse the post and swap out the URL, since you should have data about the user in your database anyway from when you first detected the user.
Maybe I am misunderstanding the use case here, but I am not sure why a platform would not direct you to its own platform for content and profiles.