Ah! #IANA's Special-Use Domain Name Registry gained an new entry.

#RFC 9476: The .alt Special-Use Top-Level Domain

“This document reserves the .alt label for use as an unmanaged pseudo-TLD namespace. The .alt label can be used in any domain name as a pseudo-TLD to signify that this is an alternative (non-DNS) namespace and should not be looked up in a #DNS context”

https://www.rfc-editor.org/info/rfc9476