I assume you mean a limited subset of 7bit ascii ? 33-126

    % host -t a $'\015'.
    1 \015:
    19 bytes, 1+0+0+0 records, response, authoritative, nxdomain
    query: 1 \015
    %

It's not as straightforward or sensible as you think. It's case insensitive; it's case preserving; and C0 control characters, SPC, and DEL are allowed. The case differentiating bits for letters are nowadays sometimes used in an attempt to foil attackers. If you want things to look back on and say "I think that X was a mistake." then forget UTF of any stripe. The DNS is full of them.

I thought DNS allowed any arbitrary byte sequence as label (up to max length limit)