how to save checkbox data for custom setting?

Your checkbox data is saved as 1 or '' if someone checked or unchecked it.

you can also verify this using var_dump($checkbox) inside custom_checkbox_field_html function

This should work.

function custom_checkbox_field_html(){
 
    $checkbox = get_option( 'disabletitle_text' );
    $is_checked = (  $checkbox != '' && $checkbox == 1 ) ? 'checked': '';

    printf(
        '<input type="checkbox" id="disabletitle_text" name="disabletitle_text" value="1" %s/>',
        esc_attr( $is_checked )
    );
 
}