Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 35748)
+++ src/wp-includes/media.php	(working copy)
@@ -1206,6 +1206,10 @@
  * @return string Converted content with 'srcset' and 'sizes' attributes added to images.
  */
 function wp_make_content_images_responsive( $content ) {
+	// Deprecation warning at function call.
+    trigger_error( 'Deprecated function called - (wp_make_content_images_responsive).', E_USER_DEPRECATED );
+    exit;
+    
 	$images = get_media_embedded_in_content( $content, 'img' );
 
 	$selected_images = $attachment_ids = array();
Index: src/wp-includes/functions.php
===================================================================
--- src/wp-includes/functions.php	(revision 35748)
+++ src/wp-includes/functions.php	(working copy)
@@ -4984,6 +4984,10 @@
  * @return string Tag RegEx.
  */
 function get_tag_regex( $tag ) {
+	// Deprecation warning at function call.
+    trigger_error( 'Deprecated function called - (get_tag_regex).', E_USER_DEPRECATED );
+    exit;
+    
 	if ( empty( $tag ) )
 		return;
 	return sprintf( '<%1$s[^<]*(?:>[\s\S]*<\/%1$s>|\s*\/>)', tag_escape( $tag ) );