How do you reset a cart under Craft Commerce 2? How do you reset or empty the cart, in Commerce 2? There is no mention of this in the documentation.

In Commerce 1, it was possible to issue a request to the commerce/cart/removeAllLineItems action to clear the cart, but this no longer works.
The commerce/cart/remove-all-line-items action (or in Craft 2 terms, commerce/cart/removeAllLineItems) was deprecated (and subsequently removed in Commerce 3), so it's up to you to build a form to explicitly delete all line items:
&lt;form method=&quot;post&quot;&gt;
    {{ csrfInput() }}
    {{ actionInput('commerce/cart/update-cart') }}

    {% for lineItem in craft.commerce.carts.cart.lineItems %}
        &lt;input type=&quot;hidden&quot; name=&quot;lineItems[{{ lineItem.id }}][remove]&quot; value=&quot;✓&quot;&gt;
    {% endfor %}

    &lt;button type=&quot;submit&quot;&gt;Empty Cart&lt;/button&gt;
&lt;/form&gt;

This snippet just loops over the current line items, and outputs a hidden input element, which sends the proper POST value to the update-cart action.
It's also possible to give your customer control over which line items are to be removed by changing the input to a check-box (and providing adequate labeling).