NIP-05 のこれか。

> Showing just the domain as an identifier

>

> Clients may treat the identifier _@domain as the "root" identifier, and choose to display it as just the . For example, if Bob owns bob.com, he may not want an identifier like bob@bob.com as that is redundant. Instead, Bob can use the identifier _@bob.com and expect Nostr clients to show and treat that as just bob.com for all purposes.

https://github.com/nostr-protocol/nips/blob/master/05.md#showing-just-the-domain-as-an-identifier