i think the rules were copied from lightning addresses. the LNURL docs explain it more clearly. https://github.com/lnurl/luds/blob/luds/16.md#:~:text=Please%20note%20that%20this%20is%20way%20more%20strict%20than%20common%20email%20addresses%20as%20it%20allows%20fewer%20symbols%20and%20only%20lowercase%20characters.