Get last item added to cart
Since multiple line item can be updated or added to the cart at the same time, it would be hard to point to a specific line item that is newest.

The line items have a dateUpdate column you could sort and query on.

{% set result = craft.query()
    .select('*')
    .from('{{%commerce_lineitems}}')
    .where({
        'orderId': cart.id
    })
    .orderBy('dateUpdated ASC')
    .one()
%}


We could look to added a lastUpdateLineItem() method to the order element in future version of Craft Commerce. You can submit the idea here: https://github.com/craftcms/commerce/issues