[Server][Client][Docs] SEP-2577: Mark Roots/Sampling/Logging @deprecated, emit notices, document migration

[chr-hertel]

Implements SEP-2577 for the MCP Spec 2026-07-28 release.

Tracked by umbrella #339.

Spec summary

Three core features are formally deprecated: Roots (server→client list/subscribe), Sampling (server→client LLM completion), and Logging (server→client log notifications). No wire-level changes — advisory only. All three remain fully functional for at least 12 months past this spec version (~through 2027-07).

PHP SDK changes

• Implementations stay. SDK MUST continue supporting all three: SetLogLevelHandler, sampling Fiber suspend/resume in Server/Protocol.php, roots in src/Client/Handler/.
• Annotate @deprecated on public API surfaces:
  • Server::builder() paths related to logging
  • Sampling-related types in Schema/
  • Client listRoots/rootsListChanged handlers
  • Use the format from SEP-2596: @deprecated Since spec 2026-07-28; will be removed no earlier than 2027-07-28
• Emit runtime deprecation notices (E_USER_DEPRECATED and/or PSR-3 notice) when these capabilities are advertised in InitializeHandler initialize result, or at server-builder time when registered.
• Docs: Add deprecation banners in docs/server-client-communication.md and docs/mcp-elements.md pointing to alternatives (stderr / OpenTelemetry for logging; tool parameters for roots).
• Consider opt-out: disableDeprecatedFeatures() builder flag.

Related

• Umbrella [2026-07-28] Governance: deprecations, lifecycle, conformance, SDK tier #339
• SEP-2596 (umbrella [2026-07-28] Governance: deprecations, lifecycle, conformance, SDK tier #339) — establishes the deprecation policy this implements