diff --git a/docs/UPGRADING.md b/docs/UPGRADING.md
index 83548e73..91fa6fe4 100644
--- a/docs/UPGRADING.md
+++ b/docs/UPGRADING.md
@@ -437,6 +437,16 @@ As a developer, one can continue using `millis()`, but migrating to `Kaleidoscop
 
 ## Breaking changes
 
+### Sketch preprocssing system
+
+We used to support the ability to amend all compiled sketches by
+adding code to
+`src/kaleidoscope_internal/sketch_preprocessing/sketch_header.h`
+and `src/kaleidoscope_internal/sketch_preprocessing/sketch_footer.h`.
+The functionality was never used by Kaleidoscope itself and frequently
+pulled the (empty) header files from the wrong copy of Kaleidoscope.
+If you need this functionality, please open a GitHub issue.
+
 ### Macros
 
 This is a guide to upgrading existing Macros code to use the new version of
diff --git a/src/kaleidoscope_internal/sketch_preprocessing/sketch_footer.h b/src/kaleidoscope_internal/sketch_preprocessing/sketch_footer.h
deleted file mode 100644
index ab5a9d18..00000000
--- a/src/kaleidoscope_internal/sketch_preprocessing/sketch_footer.h
+++ /dev/null
@@ -1,2 +0,0 @@
-// Any code that appears here is added to the bottom of the preprocessed sketch file.
-// NOLINT(build/header_guard)
diff --git a/src/kaleidoscope_internal/sketch_preprocessing/sketch_header.h b/src/kaleidoscope_internal/sketch_preprocessing/sketch_header.h
deleted file mode 100644
index 338f1217..00000000
--- a/src/kaleidoscope_internal/sketch_preprocessing/sketch_header.h
+++ /dev/null
@@ -1,3 +0,0 @@
-// Any code that appears here is added to the top of the preprocessed sketch file.
-// NOLINT(build/header_guard)
-#define KALEIDOSCOPE_SKETCH