Yep, after you refer 5 people, you can no longer refer any more. Also the subsequent rewards for them subscribing 150, 210, and 270 days only rewards the referrer for the first friend to do so. The only other method to obtain the feathers is to have someone on your friends list in-game that hasn't subscribed in 90 days and for you to use the callback campaign for 5 feathers each person.