Section 2.2 and 2.3 of RFC 5890 [1] has a good overview of the pre-IDN state of things, with reference to where these were originally specified. In particular, the definition of 'LDH label' describes a single component of a traditional DNS name:

   This is the classical label form used, albeit with some additional
   restrictions, in hostnames [RFC0952].  Its syntax is identical to
   that described as the "preferred name syntax" in Section 3.5 of RFC
   1034 [RFC1034] as modified by RFC 1123 [RFC1123].  Briefly, it is a
   string consisting of ASCII letters, digits, and the hyphen with the
   further restriction that the hyphen cannot appear at the beginning or
   end of the string.  Like all DNS labels, its total length must not
   exceed 63 octets.

I misspoke slightly in my earlier comment: it turns out the DNS protocol does allow octets of any value in a label, but the Internet domain name system -- including all the existing clients, servers, registrars, etc -- as a whole does not. Which is part of why we need a fairly complicated set of specifications to make non-ASCII domain names work.

[1] https://datatracker.ietf.org/doc/html/rfc5890#section-2.2