Milestones and labels can be used even when issues are disabled.

app/helpers/projects_helper.rb

@@ -146,7 +146,7 @@ module ProjectsHelper
       nav_tabs << feature if project.send :"#{feature}_enabled"
     end
 
-    if project.issues_enabled
+    if project.issues_enabled || project.merge_requests_enabled
       nav_tabs << [:milestones, :labels]
     end
 

app/views/projects/milestones/show.html.haml

@@ -60,6 +60,7 @@
       Participants
       %span.badge= @users.count
 
+  - if @project.issues_enabled
     .pull-right
       = link_to new_namespace_project_issue_path(@project.namespace, @project, issue: { milestone_id: @milestone.id }), class: "btn  btn-grouped", title: "New Issue" do
         %i.fa.fa-plus