Add custom text as a label in front of RSS feed icon
You can see feed-icon.html.twig

you need copy feed-icon.html.twig in core/modules/system/templates/feed-icon.html.twig to themes/yourtheme/templates/feed-icon.html.twig

in file themes/yourtheme/templates/feed-icon.html.twig

{#
/**
 * @file
 * Default theme implementation for a feed icon.
 *
 * Available variables:
 * - url: An internal system path or a fully qualified external URL of the feed.
 * - attributes: Remaining HTML attributes for the feed link.
 *   - title: A descriptive title of the feed link.
 *   - class: HTML classes to be applied to the feed link.
 *
 * @ingroup themeable
 */
#}
&lt;a href="{{ url }}"{{ attributes.addClass('feed-icon') }}&gt;
  {{ 'Subscribe to News Feed'}}
&lt;/a&gt;