Raise minimum TypeScript to 5.0.4 and drop TS 3.8 support (v9)

Start by reviewing the Type Check TypeScript 3.8 CI job, the downlevel build step, and the typesVersions mapping in packages/core/package.json. Check the alpha tracking notes in #6656 and the TypeScript version policy in the linked migration guide. Done means the floor is 5.0.4, the old down-level setup and CI job are retired or updated, and v9 migration or release notes document the user impact.

Summary

Raise the React Native SDK's minimum supported TypeScript version to 5.0.4 and drop the TypeScript 3.8 down-leveling setup.

Target: v9 (next major). This is a breaking change and will be implemented as part of the v9 major, alongside the @sentry/* JavaScript SDK v11 bump.

Background

JavaScript SDK v11 raises its minimum TypeScript to 5.0.4 and stops emitting down-leveled types (migration guide — TypeScript Version Policy). Confirmed against the alpha: @sentry/core@11.0.0-alpha.2 no longer ships the typesVersions / types-ts3.8 fallback that v10 provided for TS <5.0.

Because the RN SDK re-exports and references @sentry/core types, its effective TypeScript floor is inherited from core. The v11 core .d.ts files use TS 4.0/4.5+ syntax (labeled tuple elements; inline type import specifiers) that older TypeScript cannot parse — this is what fails the Type Check TypeScript 3.8 CI job on the alpha.2 spike (#6655). Our own downlevel-dts step only rewrites the RN SDK's own declarations; it does not (and cannot practically) rewrite the transitive @sentry/core types.

See the alpha.2 tracking notes in #6656.

Is 5.0.4 viable for React Native?

Yes. 5.0.4 is exactly the TypeScript version the RN community template pinned for RN 0.73–0.76; RN 0.77+ ships newer (^5.8.x). RN 0.71/0.72 templated TS 4.8.4, but TypeScript is decoupled from the RN version and can be bumped independently.

User impact

Compile-time only, TypeScript users only:

• Not affected: JavaScript-only users, and TypeScript users already on >= 5.0.4 (the majority).
• Affected: TypeScript users below 5.0.4 — they must bump typescript (a devDependency change, independent of their RN version). No runtime/behavior impact.

The SDK's react-native peer floor stays >= 0.65.0: the SDK still runs on old RN, but its types will require TS 5.0.4.

Tasks

• Raise the minimum supported TypeScript to 5.0.4.
• Remove the down-level setup: the downlevel build step, the downlevel-dts devDependency, and the typesVersions <4.9 → ts3.8 mapping in packages/core/package.json.
• Replace/retire the Type Check TypeScript 3.8 CI job with one pinned to the new 5.0.4 floor.
• Document in the v9 migration guide / release notes: "Minimum TypeScript raised to 5.0.4 (following @sentry/core v11); React Native support unchanged (>= 0.65). TypeScript users below 5.0.4 must upgrade TypeScript — this does not require a React Native upgrade."