diff -r 705c37f22859 -r c6a1fa42e325 mercurial/upgrade.py
--- a/mercurial/upgrade.py	Thu Jun 25 13:13:21 2020 +0530
+++ b/mercurial/upgrade.py	Fri Oct 16 18:57:55 2020 +0530
@@ -80,6 +80,7 @@
         requirements.SIDEDATA_REQUIREMENT,
         requirements.COPIESSDC_REQUIREMENT,
         requirements.NODEMAP_REQUIREMENT,
+        requirements.SHARESAFE_REQUIREMENT,
     }
     for name in compression.compengines:
         engine = compression.compengines[name]
@@ -1470,3 +1471,11 @@
                         b' New shares will be created in safe mode.\n'
                     )
                 )
+            if sharedsafe.name in removedreqs:
+                ui.warn(
+                    _(
+                        b'repository downgraded to not use share safe mode, '
+                        b'existing shares will not work and needs to'
+                        b' be reshared.\n'
+                    )
+                )