Yes my content contains Amazon's URL, but I don't insert iframe.
I don't need iframe so want to disable it.
Wordpress automatically inserts iframe.
This is the iframe for Amazon. It changes links to
and this is official Amazon Australia URL.
I don't use Amazon Australia and neither do my site readers. Also, it's big and ruin my website design.
WordPress oEmbed caused this problem.
I finally fund out that this iframe is inserted because of WordPress oEmbed function.
What a useless and annoying function! This and Gutenberg are both meddlesome.
How to disable WordPress oEmbed
Open functions.php and write this line.
remove_filter('the_content', array($wp_embed, 'autoembed'), 8);