What is the difference between:

1) a 411(d)(6) protected benefit that can be eliminated by plan amendment (like hardship distributions), and

2) a benefit that is not protected under 411(d)(6) (like ancillary life insurance)?

If they can both be eliminated by plan amendment, what's the difference?

There may be other differences too, but the regulations don't allow employer discretion for a 411(d)(6) protected benefit, even if one could amend the document to get rid of the benefit.