> No, I seemed to remember 2821 deciding that _ would be legal.

If so, it seems to have been backed out by the time it was frozen.  My
copy of 2821 says (excerpted, of course):

      Domain = (sub-domain 1*("." sub-domain)) / address-literal
      sub-domain = Let-dig [Ldh-str]
      Let-dig = ALPHA / DIGIT
      Ldh-str = *( ALPHA / DIGIT / "-" ) Let-dig

I searched, and the _ character does not appear at all in 2821.

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