This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
import type {SuspenseInstance} from './ReactFiberHostConfig';
import type {OffscreenProps} from './ReactFiberOffscreenComponent';
import invariant from 'shared/invariant';
import {
  createRootStrictEffectsByDefault,
  enableCache,
  enableStrictEffects,
  enableProfilerTimer,
  enableScopeAPI,
  enableSyncDefaultUpdates,
  allowConcurrentByDefault,
} from 'shared/ReactFeatureFlags';
import {NoFlags, Placement, StaticMask} from './ReactFiberFlags';
import {ConcurrentRoot} from './ReactRootTags';
import {
  IndeterminateComponent,
  ClassComponent,
  HostRoot,
  HostComponent,
  HostText,
  HostPortal,
  ForwardRef,
  Fragment,
  Mode,
  ContextProvider,
  ContextConsumer,
  Profiler,
  SuspenseComponent,
  SuspenseListComponent,
  DehydratedFragment,