Clarify GraphQL deprecation process for fields behind a FF

doc/api/graphql/index.md

@@ -82,6 +82,10 @@ The process is as follows:
    release post (at or prior to X.11 and X.5 releases).
 1. Fields meeting criteria are removed in X.0 or X.6.
+NOTE:
+Fields behind a feature flag and disabled by default are exempt from the deprecation process,
+and can be removed at any time without notice.
+
 ### List of removed items
 View the [fields, enums, and other items we removed](removed_items.md) from the GraphQL API.