FWIW I recently picked up a used 2016 leaf with just over 40k miles. It didn't have the 30kWh update yet, and was missing three capacity bars. After the update it's only missing one. If you can get the VIN for the one you're looking at, you can call up a dealership and have them check for that service campaign status.
You can find dealerships here https://www.nissanusa.com/dealer-locator.html
Certain dealerships are certified to deal with Leafs, on the search page, expand "Applied Filters" and check the box by "Nissan LEAF Dealer."