diff --git a/tests/external_board_native/README.md b/tests/external_board_native/README.md
index ba79ea836..86d7f3576 100644
--- a/tests/external_board_native/README.md
+++ b/tests/external_board_native/README.md
@@ -12,3 +12,7 @@ done as the 'board' module is implemented directly by the 'RIOT/boards/native'.
 
 If you are running an 'ubuntu-bionic' you could even compile the example in
 docker and run the test.
+
+*Note:* This test uses still the deprecated `BOARDSDIR` variable instead of the
+new `EXTERNAL_BOARD_DIRS` variable. This is by intent so that the CI can verify
+backward compatibility is still working as intended.