Disallow system notes for closed issuables

app/services/system_note_service.rb

@@ -385,6 +385,7 @@ module SystemNoteService
   # Returns Boolean
   def cross_reference_disallowed?(noteable, mentioner)
     return true if noteable.is_a?(ExternalIssue) && !noteable.project.jira_tracker_active?
+    return true if noteable.is_a?(Issuable) && (noteable.try(:closed?) || noteable.try(:merged?))
     return false unless mentioner.is_a?(MergeRequest)
     return false unless noteable.is_a?(Commit)

changelogs/unreleased/issue_19262.yml

+---
+title: Disallow system notes for closed issuables
+merge_request:
+author:

spec/services/system_note_service_spec.rb

@@ -418,6 +418,45 @@ describe SystemNoteService, services: true do
           to be_truthy
       end
     end
+
+    context 'when noteable is an Issue' do
+      let(:issue) { create(:issue, project: project) }
+
+      it 'is truthy when issue is closed' do
+        issue.close
+
+        expect(described_class.cross_reference_disallowed?(issue, project.commit)).
+          to be_truthy
+      end
+
+      it 'is falsey when issue is open' do
+        expect(described_class.cross_reference_disallowed?(issue, project.commit)).
+          to be_falsy
+      end
+    end
+
+    context 'when noteable is a Merge Request' do
+      let(:merge_request) { create(:merge_request, :simple, source_project: project) }
+
+      it 'is truthy when merge request is closed' do
+        allow(merge_request).to receive(:closed?).and_return(:true)
+
+        expect(described_class.cross_reference_disallowed?(merge_request, project.commit)).
+          to be_truthy
+      end
+
+      it 'is truthy when merge request is merged' do
+        allow(merge_request).to receive(:closed?).and_return(:true)
+
+        expect(described_class.cross_reference_disallowed?(merge_request, project.commit)).
+          to be_truthy
+      end
+
+      it 'is falsey when merge request is open' do
+        expect(described_class.cross_reference_disallowed?(merge_request, project.commit)).
+          to be_falsy
+      end
+    end
   end
   describe '.cross_reference_exists?' do