This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
reconcileChildFibers(workInProgress, current.child, null, renderLanes);
  }
  return fallbackChildFragment;
}
function mountDehydratedSuspenseComponent(
  workInProgress: Fiber,
  suspenseInstance: SuspenseInstance,
  renderLanes: Lanes,
): null | Fiber {
  
  
  if ((workInProgress.mode & ConcurrentMode) === NoMode) {
    if (__DEV__) {
      console.error(
        'Cannot hydrate Suspense in legacy mode. Switch from ' +
          'ReactDOM.hydrate(element, container) to ' +
          'ReactDOM.createRoot(container, { hydrate: true })' +
          '.render(element) or remove the Suspense components from ' +
          'the server rendered components.',
      );
    }
    workInProgress.lanes = laneToLanes(SyncLane);
  } else if (isSuspenseInstanceFallback(suspenseInstance)) {