Add feature flag for deployment notifications

app/models/integrations/base_chat_notification.rb

@@ -241,6 +241,7 @@ module Integrations
 
     def notify_for_ref?(data)
       return true if data[:object_kind] == 'tag_push'
+      return true if data[:object_kind] == 'deployment' && !Feature.enabled?(:chat_notification_deployment_protected_branch_filter, project)
 
       ref = data[:ref] || data.dig(:object_attributes, :ref)
       return true if ref.blank? # No need to check protected branches when there is no ref

config/feature_flags/development/chat_notification_deployment_protected_branch_filter.yml

+---
+name: chat_notification_deployment_protected_branch_filter
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/74423
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/349131
+milestone: '14.7'
+type: development
+group: group::integrations
+default_enabled: false

spec/support/shared_examples/models/concerns/integrations/slack_mattermost_notifier_shared_examples.rb

@@ -693,6 +693,16 @@ RSpec.shared_examples Integrations::SlackMattermostNotifier do |service_name|
       context 'notification enabled for all branches' do
         it_behaves_like "triggered #{service_name} service", event_type: "pipeline", branches_to_be_notified: "all"
       end
+
+      context 'when chat_notification_deployment_protected_branch_filter is disabled' do
+        before do
+          stub_feature_flags(chat_notification_deployment_protected_branch_filter: false)
+        end
+
+        context 'notification enabled only for default branch' do
+          it_behaves_like "triggered #{service_name} service", event_type: "pipeline", branches_to_be_notified: "default"
+        end
+      end
     end
   end
 end