diff --git a/CHANGES.md b/CHANGES.md
index d9afcaa52..309c34f90 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,18 @@
+Synapse 1.xx.0 (2021-xx-xx)
+===========================
+
+Removal warning
+---------------
+
+The v1 list accounts API is deprecated and will be removed in a future release.
+This API was undocumented and misleading. It can be replaced by the
+[v2 list accounts API](https://github.com/matrix-org/synapse/blob/master/docs/admin_api/user_admin_api.rst#list-accounts),
+which has been available since Synapse 1.7.0 (2019-12-13).
+
+Please check if you're using any scripts which use the admin API and replace
+`GET /_synapse/admin/v1/users/<user_id>` with `GET /_synapse/admin/v2/users`.
+
+
 Synapse 1.27.0 (2021-02-16)
 ===========================
 
diff --git a/changelog.d/9429.removal b/changelog.d/9429.removal
new file mode 100644
index 000000000..1ff9089a2
--- /dev/null
+++ b/changelog.d/9429.removal
@@ -0,0 +1 @@
+Deprecate old admin API `GET /_synapse/admin/v1/users/<user_id>`.
\ No newline at end of file