Add deprecation note for icon helper to frontend guidelines

app/helpers/icons_helper.rb

@@ -13,6 +13,7 @@ module IconsHelper
   # Right now this method simply delegates directly to `fa_icon` from the
   # font-awesome-rails gem, but should we ever use a different icon pack in the
   # future we won't have to change hundreds of method calls.
+  # @deprecated use sprite_icon to render a SVG icon
   def icon(names, options = {})
     if (options.keys & %w[aria-hidden aria-label data-hidden]).empty?
       # Add 'aria-hidden' and 'data-hidden' if they are not set in options.

doc/development/fe_guide/icons.md

@@ -22,7 +22,7 @@ Our goal is to replace one by one all inline SVG Icons (as those currently bloat
 
 ### Usage in HAML/Rails
 
-To use a sprite Icon in HAML or Rails we use a specific helper function :
+To use a sprite Icon in HAML or Rails we use a specific helper function:
 
 ```ruby
 sprite_icon(icon_name, size: nil, css_class: '')
@@ -48,6 +48,8 @@ sprite_icon(icon_name, size: nil, css_class: '')
 </svg>
 ```
 
+**Please note:** The `icon(icon_name, options: {})` helper function is deprecated and should not be used anymore.
+
 ### Usage in Vue
 
 [GitLab UI](https://gitlab-org.gitlab.io/gitlab-ui/), our components library, provides a component to display sprite icons.