--- a/mercurial/revlogutils/nodemap.py	Mon Feb 15 15:13:20 2021 +0530
+++ b/mercurial/revlogutils/nodemap.py	Wed Feb 10 17:08:34 2021 +0530
@@ -128,6 +128,14 @@
         notr._postclose[k](None)
 
 
+def delete_nodemap(tr, repo, revlog):
+    """ Delete nodemap data on disk for a given revlog"""
+    if revlog.nodemap_file is None:
+        msg = "calling persist nodemap on a revlog without the feature enabled"
+        raise error.ProgrammingError(msg)
+    repo.svfs.unlink(revlog.nodemap_file)
+
+
 def persist_nodemap(tr, revlog, pending=False, force=False):
     """Write nodemap data on disk for a given revlog"""
     if getattr(revlog, 'filteredrevs', ()):

--- a/mercurial/upgrade_utils/engine.py	Mon Feb 15 15:13:20 2021 +0530
+++ b/mercurial/upgrade_utils/engine.py	Wed Feb 10 17:08:34 2021 +0530
@@ -476,6 +476,27 @@
                 tr, unfi.manifestlog._rootstore._revlog, force=True
             )
         scmutil.writereporequirements(srcrepo, upgrade_op.new_requirements)
+    elif (
+        len(upgrade_op.removed_actions) == 1
+        and [
+            x
+            for x in upgrade_op.removed_actions
+            if x.name == b'persistent-nodemap'
+        ]
+        and not upgrade_op.upgrade_actions
+    ):
+        ui.status(
+            _(b'downgrading repository to not use persistent nodemap feature\n')
+        )
+        with srcrepo.transaction(b'upgrade') as tr:
+            unfi = srcrepo.unfiltered()
+            cl = unfi.changelog
+            nodemap.delete_nodemap(tr, srcrepo, cl)
+            # check comment 20 lines above for accessing private attributes
+            nodemap.delete_nodemap(
+                tr, srcrepo, unfi.manifestlog._rootstore._revlog
+            )
+        scmutil.writereporequirements(srcrepo, upgrade_op.new_requirements)
     else:
         with dstrepo.transaction(b'upgrade') as tr:
             _clonerevlogs(

--- a/tests/test-persistent-nodemap.t	Mon Feb 15 15:13:20 2021 +0530
+++ b/tests/test-persistent-nodemap.t	Wed Feb 10 17:08:34 2021 +0530
@@ -581,7 +581,7 @@
   plain-cl-delta:     yes    yes     yes
   compression:        zlib   zlib    zlib
   compression-level:  default default default
-  $ hg debugupgraderepo --run --no-backup --quiet
+  $ hg debugupgraderepo --run --no-backup
   upgrade will perform the following actions:
   
   requirements
@@ -593,8 +593,17 @@
     - changelog
     - manifest
   
+  beginning upgrade...
+  repository locked and read-only
+  creating temporary repository to stage upgraded data: $TESTTMP/test-repo/.hg/upgrade.* (glob)
+  (it is safe to interrupt this process any time before data migration completes)
+  downgrading repository to not use persistent nodemap feature
+  removing temporary repository $TESTTMP/test-repo/.hg/upgrade.* (glob)
   $ ls -1 .hg/store/ | egrep '00(changelog|manifest)(\.n|-.*\.nd)'
-  [1]
+  00changelog-*.nd (glob)
+  00manifest-*.nd (glob)
+  undo.backup.00changelog.n
+  undo.backup.00manifest.n
   $ hg debugnodemap --metadata
 
 
@@ -643,6 +652,8 @@
   00changelog.n
   00manifest-*.nd (glob)
   00manifest.n
+  undo.backup.00changelog.n
+  undo.backup.00manifest.n
 
   $ hg debugnodemap --metadata
   uid: * (glob)