Deprecate Zipkin exporter (#4715)

This PR intends to gather feedback on Zipkin exporter usage and possible
deprecation.

What inspired it: 

1. The transformations documented in zipkin exporter are not followed in
practice (e.g.
[otel-java](https://github.com/open-telemetry/opentelemetry-java/blob/2c379c7179d8b4eb5b391312b43c3edec3dee825/exporters/zipkin/src/main/java/io/opentelemetry/exporter/zipkin/OtelToZipkinSpanTransformer.java)
or
[zipkin-otel](https://github.com/openzipkin-contrib/zipkin-otel/blob/24308e01781431cc8a6b2c03f52dac1fd645132b/collector-http/src/main/java/zipkin2/collector/otel/http/SpanTranslator.java#L191)).
I don't see any signs of transformations in [collector
zipkinexporter](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/exporter/zipkinexporter)
These transformations were not updated despite Semantic Conventions
changes and it went unnoticed (e.g. database conventions went stable ~ 6
months ago, but the document still uses deprecated attributes).

2. Public download stats mostly show low usage. e.g.
    - python: 
-
[opentelemetry-exporter-zipkin](https://pypistats.org/packages/opentelemetry-exporter-zipkin)
- ~155k downloads last month
-
[opentelemetry-exporter-jaeger](https://pypistats.org/packages/opentelemetry-exporter-jaeger)
(deprecated and removed from the spec) - ~513k downloads last month
-
[opentelemetry-exporter-otlp](https://pypistats.org/packages/opentelemetry-exporter-otlp)
- 96M downloads last month
- .NET trends for [Zipkin, Jaeger, OTLP
exporters](https://nugettrends.com/packages?months=3&ids=OpenTelemetry.Exporter.OpenTelemetryProtocol&ids=OpenTelemetry.Exporter.Jaeger&ids=OpenTelemetry.Exporter.Zipkin)
also show zipkin having slightly less downloads than deprecated Jaeger
and ~10 times less than OTLP
    - JS shows 2.5x difference in download numbers between 
-
[@opentelemetry/exporter-zipkin](https://npm-stat.com/charts.html?package=%40opentelemetry%2Fexporter-zipkin&from=2025-09-01&to=2025-09-30)
- ~20M last month - **THIS IS QUITE HIGH** (possible explanation in
#4715 (comment))
- while
[@opentelemetry/exporter-trace-otlp-http](https://npm-stat.com/charts.html?package=%40opentelemetry%2Fexporter-trace-otlp-http&from=2025-09-01&to=2025-09-30)
along with
[@opentelemetry/exporter-trace-otlp-grpc](https://npm-stat.com/charts.html?package=%40opentelemetry%2Fexporter-trace-otlp-grpc&from=2025-09-01&to=2025-09-30)
have ~48M downloads
    - I don't have access to Java maven download stats


3. Availability of [experimental OTLP support in zipkin
server](https://github.com/openzipkin-contrib/zipkin-otel/blob/main/module/README.md)
and [zipkin exporter in
collector](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/exporter/zipkinexporter)
in beta status

4. Low activity on issues [org:open-telemetry zipkin
in:title](https://github.com/search?q=org%3Aopen-telemetry+zipkin+in%3Atitle&type=issues&s=created&o=desc)
- most of them are SIG work items including collector zipkin exporter
with just a few issues created by end users.

---------

Co-authored-by: Robert Pająk <[email protected]>

Author: lmolkova

CHANGELOG.md

@@ -11,6 +11,9 @@ release.
 
 ### Traces
 
+- Deprecate Zipkin exporter document and make exporter implementation optional.
+  ([#4715](https://github.com/open-telemetry/opentelemetry-specification/pull/4715/))
+
 ### Metrics
 
 ### Logs

spec-compliance-matrix.md

@@ -9,7 +9,7 @@ status of the feature is not known.
 
 For the `Optional` column, `X` means the feature is optional, blank means the
 feature is required, and columns marked with `*` mean that for each type of
-exporter (OTLP and Zipkin), implementing at least one of the supported
+exporter (e.g. OTLP), implementing at least one of the supported
 formats is required. Implementing more than one format is optional.
 
 ## Traces
@@ -334,8 +334,8 @@ Disclaimer: Declarative configuration is currently in Development status - work
 | **[Zipkin](specification/trace/sdk_exporters/zipkin.md)** | Optional | Go | Java | JS | Python | Ruby | Erlang | PHP | Rust | C++ | .NET | Swift |
 | Zipkin V1 JSON | X | - | + |  | + | - | - | - | - | - | - | - |
 | Zipkin V1 Thrift | X | - | + |  | [-][py1174] | - | - | - | - | - | - | - |
-| Zipkin V2 JSON | * | + | + |  | + | + | - | + | + | + | + | + |
-| Zipkin V2 Protobuf | * | - | + |  | + | - | + | - | - | - | - | - |
+| Zipkin V2 JSON | X | + | + |  | + | + | - | + | + | + | + | + |
+| Zipkin V2 Protobuf | X | - | + |  | + | - | + | - | - | - | - | - |
 | Service name mapping |  | + | + | + | + | + | + | + | + | + | + | + |
 | SpanKind mapping |  | + | + | + | + | + | + | + | + | + | + | + |
 | InstrumentationLibrary mapping |  | + | + | - | + | + | - | + | + | + | + | + |

spec-compliance-matrix/template.yaml

@@ -375,9 +375,9 @@ sections:
           - name: Zipkin V1 Thrift
             optional: true
           - name: Zipkin V2 JSON
-            optional_one_of_group_is_required: true
+            optional: true
           - name: Zipkin V2 Protobuf
-            optional_one_of_group_is_required: true
+            optional: true
           - name: Service name mapping
           - name: SpanKind mapping
           - name: InstrumentationLibrary mapping

specification/configuration/sdk-environment-variables.md

@@ -270,6 +270,8 @@ See [OpenTelemetry Protocol Exporter Configuration Options](../protocol/exporter
 
 ## Zipkin Exporter
 
+**Status**: [Deprecated](../document-status.md)
+
 | Name                          | Description                                                                        | Default                              | Type        |
 |-------------------------------|------------------------------------------------------------------------------------|--------------------------------------|-------------|
 | OTEL_EXPORTER_ZIPKIN_ENDPOINT | Endpoint for Zipkin traces                                                         | `http://localhost:9411/api/v2/spans` | [String][]  |

specification/trace/sdk_exporters/zipkin.md

@@ -4,7 +4,20 @@ linkTitle: Zipkin
 
 # OpenTelemetry to Zipkin Transformation
 
-**Status**: [Stable](../../document-status.md)
+**Status**: [Deprecated](../../document-status.md)
+
+Zipkin exporter support will be removed from OpenTelemetry specification in December
+2026.
+
+> Note: This document remains here for backwards compatibility and
+> will be removed in a future version. Existing stable Zipkin exporters MUST
+> continue to be supported for at least one year after the artifact is deprecated,
+> following the [SDK stability guarantees](/specification/versioning-and-stability.md#sdk-support).
+> Implementing a Zipkin exporter is not required for new SDKs.
+>
+> Users may use [Zipkin exporter](https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/exporter/zipkinexporter)
+> collector component or [zipkin-otel](https://github.com/openzipkin-contrib/zipkin-otel)
+> Zipkin server module.
 
 This document defines the transformation between OpenTelemetry and Zipkin Spans.
 The generic transformation [rules specified here](../../common/mapping-to-non-otlp.md) also apply. If a
@@ -69,7 +82,7 @@ not work properly. OpenTelemetry doesn't provide this consistency guarantee.
 Exporter may chose to override the value for service name based on a local root
 span to improve Zipkin user experience.
 
-*Note*, the attribute `service.namespace` MUST NOT be used for the Zipkin
+Note that the attribute `service.namespace` MUST NOT be used for the Zipkin
 service name and should be sent as a Zipkin tag.
 
 ### SpanKind