diff -r 0f5521e56b77 -r cabc840ffdee mercurial/configitems.py
--- a/mercurial/configitems.py	Mon Nov 06 10:33:40 2017 -0800
+++ b/mercurial/configitems.py	Tue Nov 07 11:22:24 2017 -0500
@@ -817,6 +817,18 @@
 coreconfigitem('sparse', 'missingwarning',
     default=True,
 )
+coreconfigitem('subrepos', 'allowed',
+    default=dynamicdefault,  # to make backporting simpler
+)
+coreconfigitem('subrepos', 'hg:allowed',
+    default=dynamicdefault,
+)
+coreconfigitem('subrepos', 'git:allowed',
+    default=dynamicdefault,
+)
+coreconfigitem('subrepos', 'svn:allowed',
+    default=dynamicdefault,
+)
 coreconfigitem('templates', '.*',
     default=None,
     generic=True,