diff --git a/serde/src/integer128.rs b/serde/src/integer128.rs
index 2f94a644..c9ff9d64 100644
--- a/serde/src/integer128.rs
+++ b/serde/src/integer128.rs
@@ -1,6 +1,11 @@
-// No longer used. Old versions of serde used this macro for supporting targets
-// that did not yet have 128-bit integer support.
 #[macro_export]
+#[deprecated = "
+This macro has no effect on any version of Serde released in the past 2 years.
+It was used long ago in crates that needed to support Rustc older than 1.26.0,
+or Emscripten targets older than 1.40.0, which did not yet have 128-bit integer
+support. These days Serde requires a Rust compiler newer than that so 128-bit
+integers are always supported.
+"]
 #[doc(hidden)]
 macro_rules! serde_if_integer128 {
     ($($tt:tt)*) => {