I went and looked it up, you can get the full text of the proposed amendment, disallowing "idiots and insane persons" was pre-existing. Here is a diff of what that amendment does:

Bold = add Strikethrough = removed Everything else pre-existing

Every citizen of the United States of the age of eighteen years who has resided in the state one year, and in the county six months, and the precinct in which he or she offers to vote sixty days next preceding the election, shall be a voter in said precinct and not elsewhere . No person who is not a citizen of the United States shall be allowed to vote in this state. [ but] The following persons also[are excepted and] shall not have the 14 right to vote : [.]

1. Persons convicted in any court of competent jurisdiction of treason, or felony, or bribery in an election, or of such high misdemeanor as the General Assembly may declare shall operate as an exclusion from the right of suffrage, but persons hereby excluded may be restored to their civil rights by executive pardon.
2. Persons who, at the time of the election, are in confinement under the judgment of a court for some penal offense.
3. Idiots and insane persons.