diff -r 08e26ef4ad35 -r 119790e1c67c mercurial/interfaces/repository.py
--- a/mercurial/interfaces/repository.py	Thu Apr 08 16:34:11 2021 +0200
+++ b/mercurial/interfaces/repository.py	Sat Apr 10 11:27:40 2021 +0200
@@ -27,14 +27,12 @@
 REVISION_FLAG_CENSORED = 1 << 15
 REVISION_FLAG_ELLIPSIS = 1 << 14
 REVISION_FLAG_EXTSTORED = 1 << 13
-REVISION_FLAG_SIDEDATA = 1 << 12
-REVISION_FLAG_HASCOPIESINFO = 1 << 11
+REVISION_FLAG_HASCOPIESINFO = 1 << 12
 
 REVISION_FLAGS_KNOWN = (
     REVISION_FLAG_CENSORED
     | REVISION_FLAG_ELLIPSIS
     | REVISION_FLAG_EXTSTORED
-    | REVISION_FLAG_SIDEDATA
     | REVISION_FLAG_HASCOPIESINFO
 )
 
@@ -457,6 +455,13 @@
         """Raw sidedata bytes for the given revision."""
     )
 
+    protocol_flags = interfaceutil.Attribute(
+        """Single byte of integer flags that can influence the protocol.
+
+        This is a bitwise composition of the ``storageutil.CG_FLAG*`` constants.
+        """
+    )
+
 
 class ifilerevisionssequence(interfaceutil.Interface):
     """Contains index data for all revisions of a file.