Plugin to blog post: https://kau-boys.com/2596/wordpress/prevent-deletion-of-post-type-items-that-are-still-used

deletion-prevention.php

<?php
/**
 * Deletion Prevention
 *
 * @package pec
 * @author  Bernhard Kau
 * @license GPLv3
 *
 * @wordpress-plugin
 * Plugin Name: Deletion Prevention
 * Plugin URI: https://gist.github.com/2ndkauboy/157ad2b7d47b540f3c0125f43dd7f18c
 * Description: Prevent deletion of Contact Form 7 items when they are embedded in posts or pages.
 * Version: 1.0.0
 * Author: Bernhard Kau
 * Author URI: https://kau-boys.de
 * Text Domain: deletion-prevention
 * License: GPLv3
 * License URI: https://www.gnu.org/licenses/gpl-3.0.txt
 */

/**
 * Register the helper taxonomy for the deletion prevention.
 */
function deletion_prevention_register_taxonomy() {
	register_taxonomy(
		'deletion_prevention_tax',
		array( 'post' ),
		array(
			'hierarchical' => false,
			'public'       => false,
		)
	);
}
add_action( 'init', 'deletion_prevention_register_taxonomy' );

/**
 * Fires once a post has been saved.
 *
 * @param int     $post_ID Post ID.
 * @param WP_Post $post    Post object.
 * @param bool    $update  Whether this is an existing post being updated.
 */
function deletion_prevention_save_post( $post_ID, $post, $update ) {
	if ( ! in_array( $post->post_type, array( 'post', 'page' ), true ) ) {
		return;
	}

	$blocks = parse_blocks( $post->post_content );

	$forms = array();
	foreach ( $blocks as $block ) {
		if ( 'contact-form-7/contact-form-selector' === $block['blockName'] ) {
			$forms[] = (string) $block['attrs']['id'];
		}
	}

	wp_set_object_terms( $post_ID, $forms, 'deletion_prevention_tax' );
}
add_action( 'save_post', 'deletion_prevention_save_post', 10, 3 );

/**
 * Prevent trashing if question is currently used in a quiz.
 *
 * @param bool|null $trash Whether to go forward with trashing.
 * @param WP_Post   $post Post object.
 */
function deletion_prevention_trash_check( $trash, $post ) {
	deletion_prevention_check( $post );
}
add_action( 'pre_trash_post', 'deletion_prevention_trash_check', 10, 2 );

/**
 * Prevent deletion if question is currently used in a quiz.
 *
 * @param bool|null $delete Whether to go forward with deletion.
 * @param WP_Post   $post Post object.
 * @param bool      $force_delete Whether to bypass the Trash.
 */
function deletion_prevention_delete_check( $delete, $post, $force_delete ) {
	deletion_prevention_check( $post );
}
add_action( 'pre_delete_post', 'deletion_prevention_delete_check', 10, 3 );

/**
 * Check if the question is used in a quiz.
 *
 * @param WP_Post $post Post object.
 */
function deletion_prevention_check( $post ) {
	$args = array(
		'post_type'      => array( 'post', 'page' ),
		'post_status'    => array(
			'publish',
			'pending',
			'draft',
			'future',
			'private',
		),
		'tax_query'      => array( // phpcs:ignore WordPress.DB.SlowDBQuery.slow_db_query_tax_query
			array(
				'taxonomy' => 'deletion_prevention_tax',
				'field'    => 'slug',
				'terms'    => (string) $post->ID,
			),
		),
		'posts_per_page' => 1,
		'fields'         => 'ids',

	);
	$posts = get_posts( $args );

	if ( ! empty( $posts ) ) {
		wp_die(
			wp_kses_post(
				sprintf(
					/* translators: %1$s: link to a filtered posts list, %2$s: link to a filtered pages list */
					__(
						'This item as it is still used in <a href="%1$s">posts</a> or <a href="%2$s">pages</a>.',
						'deletion-prevention'
					),
					admin_url( 'edit.php?post_type=post&taxonomy=deletion_prevention_tax&term=' . $post->ID ),
					admin_url( 'edit.php?taxonomy=deletion_prevention_tax&term=' . $post->ID )
				)
			)
		);
	}
}