Deprecate compatibility mode settings

This includes adding a private setting: `$_govuk-is-legacy-review-app` which is set to `true` in the legacy app stylesheets within the govuk-frontend local review app. We create this temporary dependancy so that warnings aren't fired locally but are fired for users where compatibility mode settings are true in their own sass.

app/assets/scss/app-legacy-ie8.scss

@@ -8,6 +8,10 @@ $govuk-compatibility-govukfrontendtoolkit: true;
 $govuk-compatibility-govuktemplate: true;
 $govuk-compatibility-govukelements: true;
 
+// Specify that the current stylesheet is the local legacy review app to suppress
+// deprecation warnings
+$_govuk-is-legacy-review-app: true;
+
 // Set Elements assets path
 $path: "/vendor/govuk_frontend_toolkit/assets/";
 

app/assets/scss/app-legacy.scss

@@ -8,6 +8,10 @@ $govuk-compatibility-govukfrontendtoolkit: true;
 $govuk-compatibility-govuktemplate: true;
 $govuk-compatibility-govukelements: true;
 
+// Specify that the current stylesheet is the local legacy review app to suppress
+// deprecation warnings
+$_govuk-is-legacy-review-app: true;
+
 // Set Elements assets path
 $path: "/vendor/govuk_frontend_toolkit/assets/";
 

src/govuk/settings/_compatibility.scss

@@ -16,16 +16,41 @@
 /// @group settings/compatibility
 ////
 
+/// Stylesheet is the legacy "review app"
+///
+/// Whether the stylesheet being built is the local legacy "review app", intended
+/// for testing GOV.UK Frontend with compatibility mode variables active.
+///
+/// This variable is a temporary way to suppress deprecation warnings for the
+/// compatibility mode variables and should not be set outside of app-legacy.scss
+/// and app-legacy-ie8.scss in the local app.
+///
+/// @type Boolean
+/// @access private
+/// @deprecated Will be removed in v5.0 with the rest of the compatibility mode
+/// suite of tools and settings
+
+$_govuk-is-legacy-review-app: false !default;
+
 /// Compatibility Mode: alphagov/govuk_frontend_toolkit
 ///
 /// Set this to true if you are also including alphagov/govuk_frontend_toolkit
 /// in your application.
 ///
 /// @type Boolean
 /// @access public
+/// @deprecated Will be removed in v5.0 with the rest of the compatibility mode
+/// suite of tools and settings
 
 $govuk-compatibility-govukfrontendtoolkit: false !default;
 
+@if $govuk-compatibility-govukfrontendtoolkit == true and
+  $_govuk-is-legacy-review-app == false {
+  @warn "$govuk-compatibility-govukfrontendtoolkit is deprecated. " +
+    "From version 5.0, GOV.UK Frontend will not support compatibility with the " +
+    "legacy codebase govuk_frontend_toolkit";
+}
+
 /// Compatibility Mode: alphagov/govuk_template
 ///
 /// Enabling this will:
@@ -41,9 +66,18 @@ $govuk-compatibility-govukfrontendtoolkit: false !default;
 ///
 /// @type Boolean
 /// @access public
+/// @deprecated Will be removed in v5.0 with the rest of the compatibility mode
+/// suite of tools and settings
 
 $govuk-compatibility-govuktemplate: false !default;
 
+@if $govuk-compatibility-govuktemplate == true and
+  $_govuk-is-legacy-review-app == false {
+  @warn "$govuk-compatibility-govuktemplate is deprecated. " +
+    "From version 5.0, GOV.UK Frontend will not support compatibility with the " +
+    "legacy codebase govuk_template";
+}
+
 /// Compatibility Mode: alphagov/govuk_elements
 ///
 /// Enabling this will:
@@ -56,16 +90,27 @@ $govuk-compatibility-govuktemplate: false !default;
 ///
 /// @type Boolean
 /// @access public
+/// @deprecated Will be removed in v5.0 with the rest of the compatibility mode
+/// suite of tools and settings
 
 $govuk-compatibility-govukelements: false !default;
 
+@if $govuk-compatibility-govukelements == true and
+  $_govuk-is-legacy-review-app == false {
+  @warn "$govuk-compatibility-govukelements is deprecated. " +
+    "From version 5.0, GOV.UK Frontend will not support compatibility with the " +
+    "legacy codebase govuk_elements";
+}
+
 /// Compatibility Product Map
 ///
 /// Maps product names to their settings that we can use to lookup states from
 /// within the `@govuk-compatibility` mixin.
 ///
 /// @type Map
 /// @access private
+/// @deprecated Will be removed in v5.0 with the rest of the compatibility mode
+/// suite of tools and settings
 
 $_govuk-compatibility: (
   govuk_frontend_toolkit: $govuk-compatibility-govukfrontendtoolkit,