[jwilk]

All-digit host names have been allowed since 1989.

https://tools.ietf.org/html/rfc1123#page-13

One aspect of host name syntax is hereby changed: the restriction on the first character is relaxed to allow either a letter or a digit. Host software MUST support this more liberal syntax.

[goosehonk]

Huh. Thanks! I really missed the memo there. I wonder why 1035 doesn’t mention that it is updated-by 1123.