There are varying degrees of "closed", in a manner of speaking. The first year, I got my original flock from Ideal via the feedstore. At about 11 months, I bought one rooster, my Hawkeye. He was under quarantine for about five weeks and he was the only started bird I have ever bought It was stressful, but he was clean, health-wise.
The second year, I got 2 Brahmas from Ideal as days old chicks from the same feedstore.
The third year, I added four Ideal chicks as days old to a broody hen's hatchling. They came directly from Ideal about the day they arrived at the feedstore. BTW, this feedstore has one of those stack brooders that is cleaned regularly and no customer is allowed to pick up or touch the chicks, like at some places. Then, I added two more from Ideal, same thing.
The only other bird added here that did not come from the hatchery or hatching eggs from reputable breeders I trust was an 8 week old pullet, a gift from a friend, who hatched her from breeder eggs. She was under quarantine for a very long time. She was/is perfectly healthy.
I have purchased my last feedstore chicks. I have not and will not buy chicks from any private individual. I have gotten hatching eggs from two or three long-time breeders here on BYC that I trust, who adhere to the same philosophy about health that I do.
I do not show birds, do not buy birds from anyone anywhere for any reason, I do not trade birds. The feedstore owner sometimes takes some of her Ideal shipment home to raise up to POL and I will NEVER buy any of those birds because they left the brooder.
So, in summary, Hawkeye and the gift pullet from a trusted friend who has never had flock illness are the only birds who have come here that were not days-old from the hatchery or hatched from breeder's eggs (quality breeders, not just someone who has a backyard flock) No bird who leaves my property can return.
That is my flock history and my policy on acquiring birds. You may call that closed, you may not. I believe I have had a much more closed flock than most people here have. Everyone gets their first birds from somewhere and later on, adds more from somewhere, or their flock would eventually die out or become extremely inbred over time. I've always been honest in where and how I acquired my birds and I do practice what I preach.
For those of you who have not had closed flocks in the past, you can close your flock at any time and keep it that way.