docs(observability): deprecate trace input and output

[marliessophie]

Summary

• explain that trace-level input and output are deprecated in Langfuse v4
• direct users to observation input and output for browsing, evaluation, and summaries
• retain temporary compatibility guidance for existing trace-level evaluators

Verification

• git diff --check
• documentation content was previously validated in PR docs(evals): update trace-level evaluator upgrade guide #3323; this PR only separates it for independent review and merge

Greptile Summary

This PR updates the trace input/output FAQ for Langfuse v4. The main changes are:

• Explains the observation-centric data model.
• Recommends observation-level browsing and evaluation.
• Updates troubleshooting for flushing, filtering, decorators, and OpenTelemetry.
• Retains deprecated trace I/O methods for legacy evaluators.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed documentation.

_{Reviews (1): Last reviewed commit: "docs(observability): deprecate trace inp..." | Re-trigger Greptile}

Context used:

• Rule used - Move imports to the top of the module instead of p... (source)

Learned From
langfuse/langfuse-python#1387

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Project	Deployment	Actions	Updated (UTC)
langfuse-docs	Ready	Preview, Comment	Jul 22, 2026 10:13am

[github-actions]

@claude review

[claude]

LGTM — straightforward documentation rewrite with no functional code changes.

Extended reasoning...

Overview

This PR rewrites a single FAQ page (content/faq/all/empty-trace-input-and-output.mdx) to reflect that trace-level input/output is deprecated in Langfuse v4, redirecting readers to observation-level input/output for browsing, evaluation, and OTEL attribute mapping, while retaining a clearly-marked temporary compatibility section for legacy trace-level evaluators.

Security risks

None — this is a documentation-only change with no code, config, or auth surface touched.

Level of scrutiny

Low. The change is confined to prose and code examples in one markdown file; it doesn't touch application logic, build config, or shared components. I verified the internal links referenced in the rewritten content resolve to real files (/docs/v4, /docs/observability/data-model, /docs/observability/sdk/advanced-features, /integrations/native/opentelemetry, /faq/all/llm-as-a-judge-migration), and there's no md-override counterpart for this route to keep in sync. The file has exactly one H1 and properly nested subsections.

Other factors

The PR description notes this content was already validated in a prior PR (#3323) and is merely being split out for independent review/merge, consistent with the diff being a clean content substitution. The bug-hunting system found no issues, and the one candidate issue it considered (dangling OTEL attribute-mapping inconsistency) was checked and refuted against the current opentelemetry.mdx content.