This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
mode |= DebugTracingMode;
        break;
      case REACT_STRICT_MODE_TYPE:
        fiberTag = Mode;
        mode |= StrictLegacyMode | StrictEffectsMode;
        break;
      case REACT_PROFILER_TYPE:
        return createFiberFromProfiler(pendingProps, mode, lanes, key);
      case REACT_SUSPENSE_TYPE:
        return createFiberFromSuspense(pendingProps, mode, lanes, key);
      case REACT_SUSPENSE_LIST_TYPE:
        return createFiberFromSuspenseList(pendingProps, mode, lanes, key);
      case REACT_OFFSCREEN_TYPE:
        return createFiberFromOffscreen(pendingProps, mode, lanes, key);
      case REACT_LEGACY_HIDDEN_TYPE:
        return createFiberFromLegacyHidden(pendingProps, mode, lanes, key);
      case REACT_SCOPE_TYPE:
        if (enableScopeAPI) {
          return createFiberFromScope(type, pendingProps, mode, lanes, key);
        }
       
      case REACT_CACHE_TYPE:
        if (enableCache) {
          return createFiberFromCache(pendingProps, mode, lanes, key);
        }
       
      default: {
        if (typeof type === 'object' && type !== null) {
          switch (type.$$typeof) {
            case REACT_PROVIDER_TYPE: