How can I exclude specific nodes from the sitemap programmatically?

Go the same issue as #11, I had to use a form alter to achieve it.

In my case I had a field_enable_redirect, but you can change to use whatever custom behavior you need.

function mymodule_form_alter(&$form, FormStateInterface $form_state, $form_id) {
    if ($form_state->getFormObject() instanceof EntityFormInterface) {
        $entity = $form_state->getFormObject()->getEntity();

        if ($entity instanceof ContentEntityInterface && !$entity->isNew() && $entity->hasField('field_enable_redirect')) {
            $form['actions']['submit']['#validate'][] = '::validateForm';
            $form['actions']['submit']['#validate'][] = 'admin_helper_redirect_form_validate';
        }
    }
}

function mymodule_redirect_form_validate(&$form, FormStateInterface $form_state) {
    $entity = $form_state->getFormObject()->getEntity();

    if ($entity instanceof ContentEntityInterface && !$entity->isNew() && $entity->hasField('field_enable_redirect')) {
        $field_enable_redirect = $form_state->getValue('field_enable_redirect');

        // Change the value of the sitemap index setting when there is a
        // redirect.
        if ($field_enable_redirect['value'] == 1) {
            $sitemap_index = $form_state->getValue('simple_sitemap');
            $sitemap_index['default']['index'] = 0;
            $form_state->setValue('simple_sitemap', $sitemap_index);
        }
    }
}

The issue described in #11 is due to the module simple_sitemap overwritting the variant with the value of the form_state. Basically the node gets excluded in your hook_entity_presave() and then gets included again in the EntityFormHandler.php of the module.

#14 does the trick, but it means your node will be excluded two times : one in the EntityFormHandler.php and one in your hook_entity_presave().

One solution is to unset the value :)
The function of @phjou then becomes :

/**
 * Exclude from sitemap when there is a redirection.
 */
function mymodule_redirect_form_validate(&$form, FormStateInterface $form_state) {
    $entity = $form_state->getFormObject()->getEntity();

    if ($entity instanceof ContentEntityInterface && !$entity->isNew() && $entity->hasField('field_enable_redirect')) {
        // Exclusion will be processed in precreate function.
        // Clear the value so that it does not get overwritten by simple_sitemap module.
        if ($form_state->getValue('field_enable_redirect')['value']) {
            $form_state->unsetValue('field_enable_redirect')
        }
    }
}