add_filter( 'mastodon_api_account_followers', function ( $followers, $user_id, $request ) {
   $account     = new Entity\Account();
   $account->id = $user_id;

   $followers[] = $account;

   return $followers;
}, 10, 3 );

\apply_filters( 'mastodon_api_account_followers', array(), $user_id, $request )