code-embed.php

<?php

function embed_code( $content ) {

	global $post;

	$i = 1;
	while ( $i < 6 ) {

		$code = 'CODE' . $i;

		$html    = get_post_meta( $post->ID, $code, false );
		$content = str_replace( '{{' . $code . '}}', $html[0], $content );
		$i++;
	}
	return $content;

}
add_filter( 'the_content', 'embed_code' );

Super-simple method of embedding code into a WordPress post without the editor modifying it.

Once the above code is in place, add a custom field named CODEx, where x is a number between 1 and 5. The field value is your code to be embedded. Then, in your post, add {{CODEx}} where you wish the code to appear (again, replace the x with the appropriate number).

** VIP full coding-standards compliant **

Full detail here... https://artiss.blog/2018/04/embedding-code-in-your-wordpress-posts/