@renchap @tchambers It is opt in, yes.
The master list of PieFed instances is retrieved from the fediverse.observer API. Then an API endpoint on every instance in the list is queried to see if they opted in and to get information about the instance that fediverse.observer does not have.
This happens once per day.
Instance admins can choose to filter out any other instance and defederated ones are automatically filtered.