Yeah I see it. If I understand correctly, they got a list of naughty words somewhere and made them part of "reserved identifiers". So you cannot come and create an account @hitler. That does not mean it's a forbidden word as nostr:npub180cvv07tjdrrgpa0j7j7tmnyl2yr6yr7l8j4s3evf6u64th6gkwsyjh6w6 suggests.