diff -r e51392acd70c -r 21b3e6116bd1 mercurial/requirements.py
--- a/mercurial/requirements.py	Mon May 03 12:30:14 2021 +0200
+++ b/mercurial/requirements.py	Mon May 03 12:30:24 2021 +0200
@@ -41,10 +41,6 @@
 # This is why once a repository has enabled sparse-read, it becomes required.
 SPARSEREVLOG_REQUIREMENT = b'sparserevlog'
 
-# A repository with the sidedataflag requirement will allow to store extra
-# information for revision without altering their original hashes.
-SIDEDATA_REQUIREMENT = b'exp-sidedata-flag'
-
 # A repository with the the copies-sidedata-changeset requirement will store
 # copies related information in changeset's sidedata.
 COPIESSDC_REQUIREMENT = b'exp-copies-sidedata-changeset'