Add Deprecation headers on deprecated endpoints (#34262)

Co-authored-by: Damien Mathieu <42@dmathieu.com>

spec/requests/api/v1/trends/tags_spec.rb

@@ -15,6 +15,8 @@ RSpec.describe 'API V1 Trends Tags' do
           .and not_have_http_link_header
         expect(response.content_type)
           .to start_with('application/json')
+        expect(response.headers['Deprecation'])
+          .to be_nil
       end
     end
 
@@ -31,6 +33,8 @@ RSpec.describe 'API V1 Trends Tags' do
           .and have_http_link_header(api_v1_trends_tags_url(offset: 2)).for(rel: 'next')
         expect(response.content_type)
           .to start_with('application/json')
+        expect(response.headers['Deprecation'])
+          .to be_nil
       end
 
       def prepare_trends

spec/requests/api/v1/trends_spec.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'deprecated API V1 Trends Tags' do
+  describe 'GET /api/v1/trends' do
+    context 'when trends are disabled' do
+      before { Setting.trends = false }
+
+      it 'returns http success' do
+        get '/api/v1/trends'
+
+        expect(response)
+          .to have_http_status(200)
+          .and not_have_http_link_header
+        expect(response.content_type)
+          .to start_with('application/json')
+        expect(response.headers['Deprecation'])
+          .to start_with '@'
+      end
+    end
+
+    context 'when trends are enabled' do
+      before { Setting.trends = true }
+
+      it 'returns http success' do
+        prepare_trends
+        stub_const('Api::V1::Trends::TagsController::DEFAULT_TAGS_LIMIT', 2)
+        get '/api/v1/trends'
+
+        expect(response)
+          .to have_http_status(200)
+          .and have_http_link_header(api_v1_trends_tags_url(offset: 2)).for(rel: 'next')
+        expect(response.content_type)
+          .to start_with('application/json')
+        expect(response.headers['Deprecation'])
+          .to start_with '@'
+      end
+
+      def prepare_trends
+        Fabricate.times(3, :tag, trendable: true).each do |tag|
+          2.times { |i| Trends.tags.add(tag, i) }
+        end
+        Trends::Tags.new(threshold: 1).refresh
+      end
+    end
+  end
+end