"Dust birds" (Cockatiels, Cockatoos, African Greys) are known for dropping feathers slowly over time unlike "oil birds" (Amazons, Macaws, etc) which tend to drop many feathers in a shorter period.

If a 'tiel has no tailfeathers, it's not molting. It could be from feather plucking, severe feather shredding (which usually ends up making the feathers look very short, but you can usually still tell that a bird has them), or possibly damaging the feathers on cage bars...But if that were the case you'd probably still see some tailfeather. It would just be damaged.

Also, birds have what we can only assume is a defense mechanism that involves the tailfeathers. Lots of small bird owners who have at one point needed to chase a bird down have seen this. I've never heard of it happening in a bird larger than a Ringneck Parakeet. If you accidentally grab onto the tailfeathers, they're just "released". You don't need to yank. If a frightened bird gets it's tailfeathers grabbed, they'll all come out. I had this happen to me with a Budgie once.

As it was said, the damage done from plucking can end up damaging the feather folicles, and in this case they won't grow back. I've heard of it happening in as little as four years, but it usually takes much longer.