diff --git a/aip/general/0157.md b/aip/general/0157.md
index fb9066a0bc..69993669a7 100644
--- a/aip/general/0157.md
+++ b/aip/general/0157.md
@@ -78,11 +78,21 @@ enum BookView {
 
 ### Read masks as a request field
 
-**Warning:** Read masks as a single field on the request message, for example: `google.protobuf.FieldMask read_mask` are **DEPRECATED**.
+**Warning:** Read masks as a single field on the request message, for example: `google.protobuf.FieldMask read_mask` are **DEPRECATED**. Users and Google APIs **must** instead consult the `fields`
+[system parameter](https://cloud.google.com/apis/docs/system-parameters).
+
+## Rationale
+
+### Discontinuing `read_mask`
+
+Long before the `read_mask` request field was introduced, Google APIs at a
+system level supported the same functionality via the system parameter `fields`.
+So, `read_mask` as a request field was discontinued to dedupe the functionality.
 
 ## Changelog
 
-- **2023-05-09**: Fix top-level enum example and link to AIP-126.
+- **2025-05-12:** Add historical rationale for read_mask discontinuation.
+- **2023-05-09:** Fix top-level enum example and link to AIP-126.
 - **2022-03-14:** Updated guidance on default value and how to specify a read mask.
 - **2021-10-06:** Updated the guidance with system parameters.
 - **2021-03-04:** Added guidance for conflicting view enums.