This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
if (__DEV__) {
    if (tailMode !== undefined && !didWarnAboutTailOptions[tailMode]) {
      if (tailMode !== 'collapsed' && tailMode !== 'hidden') {
        didWarnAboutTailOptions[tailMode] = true;
        console.error(
          '"%s" is not a supported value for tail on <SuspenseList />. ' +
            'Did you mean "collapsed" or "hidden"?',
          tailMode,
        );
      } else if (revealOrder !== 'forwards' && revealOrder !== 'backwards') {
        didWarnAboutTailOptions[tailMode] = true;
        console.error(
          '<SuspenseList tail="%s" /> is only valid if revealOrder is ' +
            '"forwards" or "backwards". ' +
            'Did you mean to specify revealOrder="forwards"?',
          tailMode,
        );
      }
    }
  }
}
function validateSuspenseListNestedChild(childSlot: mixed, index: number) {
  if (__DEV__) {
    const isAnArray = isArray(childSlot);
    const isIterable =
      !isAnArray && typeof getIteratorFn(childSlot) === 'function';
    if (isAnArray || isIterable) {
      const type = isAnArray ? 'array' : 'iterable';
      console.error(
        'A nested %s was passed to row #%s in <SuspenseList />. Wrap it in ' +