Deprecate OpenTracing compatibility requirements

[adp2201]

Summary

Deprecate OpenTracing compatibility requirements in the specification as Stage 1 of issue #4849.

This PR changes the OpenTracing compatibility spec document status to deprecated,
adds a deprecation note, and updates wording to make it explicit that new
OpenTracing compatibility implementations are no longer required by this
specification.

What changed

• specification/compatibility/opentracing.md
  • Status changed from Stable to Deprecated
  • Added top-level deprecation note:
    • OpenTracing compatibility requirements are deprecated
    • existing shims MAY continue for backwards compatibility
    • new compatibility implementations are not required
    • removal criteria/timeline deferred to a separate discussion
  • Updated one normative paragraph to historical framing, aligned with deprecation
  • Added explicit sentence that this section remains for migration/backwards compatibility guidance
• CHANGELOG.md
  • Added Unreleased/Compatibility entry for deprecating OpenTracing compatibility requirements

Context and precedent

• Addresses: Deprecate OpenTracing compatibility requirements #4849
• Follows the staged approach discussed in Deprecate OpenTracing compatibility requirements #4849 comments
• Uses the same deprecation pattern/intent as prior deprecation work in Deprecate Zipkin exporter #4715 (Zipkin exporter)

Scope

This PR is intentionally narrow (Stage 1 only): deprecate requirements in spec text.

Non-goals

• No immediate removal of OpenTracing compatibility content
• No language-specific implementation mandates
• No removal timeline policy in this PR (to be handled in follow-up discussion)

Validation

Local checks run:

• make markdownlint ✅
• make markdown-toc && git diff --exit-code ':*.md' ✅
• make cspell ✅

Not run locally due environment limitation:

• make markdown-link-check (requires Docker daemon)

Notes for reviewers

This is documentation/spec wording only and does not introduce API/SDK interfaces.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: adp2201 / name: Amol Patil (4326cf4, a2b1a12)

[adp2201]

Comms follow-up opened in parallel as a draft blog PR: open-telemetry/opentelemetry.io#9386 (tracking issue: open-telemetry/opentelemetry.io#9385). I will keep it aligned with any wording updates requested here.

[carlosalberto]

I'd like us to go with the Zipkin approach and give ourselves a year for support plus mentioning a date (specially important as this is a Specification, not an OTEP that could be more vague).

[adp2201]

thanks, that makes sense.
Let’s align this with the Zipkin deprecation pattern in this PR:

• state that OpenTracing compatibility requirements are deprecated as of March 2026,
• state that spec removal will happen no earlier than March 2027.

We originally planned to add this in a follow-up phase, but it makes sense to frontload this clarification in Stage 1 for spec-level clarity.

I’ll update the PR with these changes.