fix(AIP-157): reinstate read mask as historical guidance

aip/general/0157.md

@@ -78,10 +78,36 @@ 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 mask as a single, explicit field on the request message is
+**DEPRECATED** for Google APIs. The [system parameter](#field-masks-parameter)
+**must** be used instead. The following guidance is for the benefit of existing
+Google and any non-Google usage.
+
+An API **may** support read masks as a single field on the request message:
+`google.protobuf.FieldMask read_mask`.
+
+- The read mask **must** be a `google.protobuf.FieldMask` and **should** be
+  named `read_mask`.
+- The field mask **should** be optional:
+  - An explicit value of `"*"` **should** be supported, and **must** return all
+    fields.
+  - If the field mask parameter is not provided, all fields **must** be
+    returned.
+- An API **may** allow read masks with non-terminal repeated fields (unlike
+  update masks), but is not obligated to do so.
+
+## Rationale
+
+### Deprecating `read_mask` in request messages
+
+As mentioned, Google API infrastructure implements a service-wide response
+field filtering mechanism, so there is no need for individual API methods to
+specify a `read_mask` in their request schema. Doing so is both redundant and a
+potential point of conflict for the client or service.
 
 ## Changelog
 
+- **2025-06-16**: Reinstate read mask guidance as historical/external reference.
 - **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.