Back in the day all major browser provided their own "feed available" icon so including an extra icon on your site was pointless. The browser's icon was consist

Back in the day all major browser provided their own "feed available" icon so including an extra icon on your site was pointless. The browser's icon was consistent across sites instead of the user needing to look around for a feed link on your site. Of course now most popular browsers don't show this icon so it is more important to advertise it yourself. (Although there are extensions to bring this icon back)