Can nutritional yeast flakes be used as a bread leavening

Your question: is it good to use. My answer: yes.

1. You say it smells and looks normal. That's an important clue.

2. We are talking about a dry product. No moist means no growth, no toxins...

3. Best before date. Wikipedia:

Most shelf life dates are used as guidelines based on normal and expected handling and exposure to temperature. Use prior to the expiration date does not necessarily guarantee the safety of a food or drug, while a product is not always dangerous nor ineffective after the expiration date

There are various types of bread without yeast. Unleavened Jewish Passover bread, for one, and soda bread for another. Unleavened bread doesn't have the spongy texture.

Basically, yeast has two uses in baking. 1. The spongelike structure and 2. Taste.