Editor: Add admin notice for Gutenberg block markup in content

src/wp-admin/edit-form-advanced.php

@@ -469,6 +469,19 @@
 		'additional_classes' => array( 'error', 'hidden' ),
 	)
 );
+
+if (
+	isset( $post->post_content ) &&
+	str_contains( (string) $post->post_content, '<!-- wp:' )
+) :
+	wp_admin_notice(
+		__( 'This content includes blocks, and some of the block content might not display in the Visual editor. Please review the content carefully before overwriting or deleting it.' ),
+		array(
+			'type'        => 'warning',
+			'dismissible' => true,
+		)
+	);
+endif;
 ?>
 <form name="post" action="post.php" method="post" id="post"
 <?php