What builds the "Vanilla Add-ons by this user" panel?

resplence

Is it custom-built specially for this forum? Can a similar functionality be accomplished with a Nugget? For example, displaying the user's "participated threads" or comments?

lech

Currently, it's a custom extension by Mark for the Community forum here only. You can however create a similaradd-on of your choosing if you read up on how to create your own add-on which can be located in the documentation.