[flags] Clean up enableHiddenSubtreeInsertionEffectCleanup

[everettbu]

Mirror of facebook/react#35918
Original author: hoxyq

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Been enabled in stable for quite a while, also rolled out at Meta.

[greptile-apps]

Greptile Summary

Cleans up the enableHiddenSubtreeInsertionEffectCleanup feature flag, which has been enabled in stable and rolled out at Meta. The flag guarded a fix ensuring insertion effect cleanup runs on hidden subtrees during deletion. The PR removes the flag declaration from all 9 feature flag files (main + forks), inlines the enabled code path in ReactFiberCommitWork.js, and simplifies test assertions by removing conditional gate() checks — always expecting the enabled behavior. No references to the flag remain in the codebase.

• Removed enableHiddenSubtreeInsertionEffectCleanup from ReactFeatureFlags.js and all fork variants
• Inlined the enabled path in commitDeletionEffectsOnFiber so commitHookEffectListUnmount(HookInsertion, ...) always runs unconditionally during deletion
• Simplified test assertions across 3 test files to always expect insertion effect cleanup behavior

Confidence Score: 5/5

• This PR is a straightforward feature flag cleanup with no behavioral changes — safe to merge.
• The flag was already true in the main feature flags file and had been rolled out in stable and at Meta. The PR is a mechanical removal: delete the flag, inline the enabled code path, and simplify tests. No logic changes, no new code, and grep confirms zero remaining references.
• No files require special attention.

Important Files Changed

Filename	Overview
packages/react-reconciler/src/ReactFiberCommitWork.js	Removes enableHiddenSubtreeInsertionEffectCleanup import and flag guard, inlining the enabled code path so commitHookEffectListUnmount(HookInsertion, ...) always runs on deletion regardless of offscreenSubtreeWasHidden.
packages/shared/ReactFeatureFlags.js	Removes the enableHiddenSubtreeInsertionEffectCleanup flag declaration (was set to true).
packages/react-reconciler/src/tests/ReactHooksWithNoopRenderer-test.js	Removes gate('enableHiddenSubtreeInsertionEffectCleanup') conditional, always expecting the useInsertionEffect warning.
packages/react-reconciler/src/tests/ReactSuspenseEffectsSemantics-test.js	Removes 4 conditional gate(flags => flags.enableHiddenSubtreeInsertionEffectCleanup) checks, always including the insertion destroy logs in assertions.
packages/react-reconciler/src/tests/useEffectEvent-test.js	Removes outer gate('enableHiddenSubtreeInsertionEffectCleanup') conditional wrapping, keeping the inner enableViewTransition/enableEffectEventMutationPhase gate logic.

_{Last reviewed commit: d17bcda}

[greptile-apps]

_{13 files reviewed, no comments}

_{Edit Code Review Agent Settings | Greptile}

[everettbu]

Upstream PR was closed or merged. Code is synced via branch mirror.