This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
lastContentRow: null | Fiber,
  tailMode: SuspenseListTailMode,
): void {
  const renderState: null | SuspenseListRenderState =
    workInProgress.memoizedState;
  if (renderState === null) {
    workInProgress.memoizedState = ({
      isBackwards: isBackwards,
      rendering: null,
      renderingStartTime: 0,
      last: lastContentRow,
      tail: tail,
      tailMode: tailMode,
    }: SuspenseListRenderState);
  } else {
    
    renderState.isBackwards = isBackwards;
    renderState.rendering = null;
    renderState.renderingStartTime = 0;
    renderState.last = lastContentRow;
    renderState.tail = tail;
    renderState.tailMode = tailMode;
  }
}
function updateSuspenseListComponent(
  current: Fiber | null,
  workInProgress: Fiber,
  renderLanes: Lanes,
) {
  const nextProps = workInProgress.pendingProps;