I am looking to understand if Shopify Plus is required for a headless implementation. Note, I understand that headless can mean different things to different people. In my case, I mean a full site as opposed to a custom app to provide a single function/feature.
It seems as it is not required as all customers have access to the Storefront API. However, I can't find much or any information on it beyond what is provided in the Shopify Plus area of the website https://www.shopify.com/plus/solutions/headless-commerce
Are there bits and pieces of access that is different depending on Shopify vs Shopify plus?
No, in fact you can do Headless ecommerce with just Shopify Lite subscription ($9/mo) by utilizing Storefront API.
What Shopify Plus provides does not directly benefit headless ecommerce as much except for some additional cart/checkout manipulation with Shopify Scripts and also Checkout styling (if you redirect to Shopify checkout).