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.
I don't think they are a rare breed in the US now because of hatcheries but good quality birds are still rare. same thing here in Australia, poor quality ones are becoming easier to find but quality birds are like finding a diamond in beach sand