Shadow: ✨ [PANA-6624] Enable the new serialization algorithm by default (v1)

[allspain]

Shadow Review — PR #4507 v1

Source: DataDog/browser-sdk#4507
Commit: f00c970cbca8ac3bd93599ee40d52a3fb412cff6
Timestamp: 2026-04-20T17:22:52Z

---

Original PR Description

Motivation

This PR makes the new session replay serialization algorithm the default in V7.

Changes

• The experimental feature flags have been removed.
• The internal API functions now return records in the new format. (I've confirmed that all callers can handle this.)
• The recording code no longer directly depends on the old serialization code, allowing it to be excluded from the bundle. (Some tests do still depend on it, so the code can't be removed yet, but we'll get there.)
• Some session replay tests had two variants: one version tested the old format, and one version tested the new one. In cases where the variant testing the old format no longer works (e.g. the E2E tests, which relied on the removed experimental feature flags), that variant has been removed.
• Many other session replay tests have been updated.

Test instructions

It should be enough to test recording with yarn dev in the usual way.

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file