See: https://calderaforms.com/doc/caldera_forms_render_get_field/

caldera_forms_render_get_field.php

<?php
/**
 * Set a specific Caldera Forms field to NOT be required
 */
add_filter( 'caldera_forms_render_get_field', function( $field ){
    //IMPORTANT: Change this to your field's ID
    if( 'fld1234' === $field[ 'ID' ] ){
        $field[ 'required' ] = 0;
    }

    return $field;
});