Arrow button on AdSense ads
Today, an arrow button (also called a Nessie arrow) is a standard part of a textual Google AdSense ad:
A clickable arrow icon was first introduced in September 2012 for mobile devices and in December 2012 for desktop:
The buttons significantly increased click-through rate, but soon it was discovered that many users click on these arrows by mistake thinking it's a site navigation. In March 2013 Google removed arrows on many sites with the following explanation: "Our systems have determined that some ad elements in text ads implemented on your site are not performing well for users, advertisers, and/or the publish network, therefore, the arrow button may not display on your ads moving forward. Different ad elements may or may not appear in text ads depending on what is deemed best performing for users, advertisers, and publishers."
These algorithms that detect the rate of accidental clicks for ads with an arrow button are still working up to this day and automatically disable the buttons when needed. If the buttons are disabled on your site, you need to change ads layout (add more space around ads, reposition ads or reposition actual site navigation). After a change that reduces accidental ad clicks, the buttons should come back in a few days.
How the button looks changed through the years. In 2015 the arrow became simpler:
Now the button is round on mobile and just a circle on desktop:
You can automatically monitor arrow buttons presence on your site and how they look like with the Ads Monitor service.