This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
workInProgress.flags &= StaticMask | Placement;
  
  const current = workInProgress.alternate;
  if (current === null) {
    
    workInProgress.childLanes = NoLanes;
    workInProgress.lanes = renderLanes;
    workInProgress.child = null;
    workInProgress.subtreeFlags = NoFlags;
    workInProgress.memoizedProps = null;
    workInProgress.memoizedState = null;
    workInProgress.updateQueue = null;
    workInProgress.dependencies = null;
    workInProgress.stateNode = null;
    if (enableProfilerTimer) {
      
      
      workInProgress.selfBaseDuration = 0;
      workInProgress.treeBaseDuration = 0;
    }
  } else {