{{ feed_icons }} has been removed from page.html.twig, there is no replacement in the theme layer.
A "feature" that has existed in Drupal for more than ten years, is the automatic display of feed icons for RSS feeds on the current page. Sadly, this was not a configurable feature, and was hardcoded deep into the system, to the point where you'd have to hack your page.html.twig template (page.tpl.php in Drupal 7 and earlier) to get rid of it. You could not even change its location.
Worse still: if you had multiple feeds on a certain page, multiple feed icons would be rendered next to each other, without a margin in between: extremely ugly. Not to mention you can't figure out which feed icon is for which feed (unless you hover over it… which then again doesn't work on touch devices).
As such, it was a crappy site builder/developer experience, because you couldn't turn it off or move it elsewhere without overriding a template. It was also a crappy themer experience, because you can't change much about {{ feed_icons }}.
Hence it should be a block. We already have a block for that: the Syndicate block. But the Syndicate block only supports the "main" RSS feed, not any specific feeds (e.g. for taxonomy terms). Of course, that doesn't help you if you have multiple feeds. You'd have to create a custom block. But almost every site would want to do that nowadays already anyway: most sites have some sort of "Follow" block nowadays, that lists the site's social media profiles (Twitter, Facebook, and so on), plus the available feeds. Since social media keep evolving (new logos) and changing (new social media appear, less popular ones disappear), it'd be inappropriate anyway for Drupal to provide such a block.
Hence, we've removed the built-in ability to automatically create feed icons for all feeds on a certain page. Instead, Views with attached feeds will now show a feed icon as part of the View (which makes the location/appearance controllable again) and recommend you to create a "Follow" block that suits your site's needs.
(But: Views' long-supported ability to show feed icons is actually broken right now, once #2359161: Feed icons missing in views blocks and pages lands, that will work again — this time with test coverage!)
