They are not rare. Most breeds were "rare" at some point though (so if you google a breed, 5 years ago it may have been very rare but today it is not). Generally the rare ones today are rare either because they're not widely bred, regional (like a Sulmtaler, etc.) or because of undesirable traits (like bantam eggs, skittish birds, defenseless, etc.), or even they're just a fad breed like Ayam Cemanis.
That said, *right now* most all chickens are rare because the hatcheries were cleared out by Covid frantic buyers... but they will likely all have very healthy stocks this coming spring... in fact I would suspect a flip the other direction where the hatcheries produce too many expecting the same panic buying as they saw this year.