My Wordpress suddenly start inserting Amazon ads iframe.
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

https://read.amazon.com.au/kp/embed～

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);

Problem solved!