Before we continue, let's be clear that battery capacity loss is not unique to the Bolt. The Nissan Leaf, especially units that were used in warm climates, have big problems when it was first launched, and Nissan came up with a number of solutions (better replacement packs, and replacement pack lease options) and the controversy died down. Tesla Model S drivers, too, have had to deal with capacity loss.
So, with that in mind, let's turn back to the Bolt EV. On page 322 of the car's manual (PDF available here) it says:
While a 10-percent range decrease won't be much to worry about, forty percent of 238 miles is 95.2, which would leave just 143 miles in a 2017 Bolt in 2025. Any eight-year-old car will have problems compared to a new model, but there are two big questions here. First, since the Bolt EV is the one that GM wants to promote to a much wider audience than the typical EV buyer – we have to wonder if this sort of range decrease will be acceptable to the masses. Second, is the phrasing "as much as 40%" something that the engineers predict will happen or is that just the lawyers covering the company's collective behind?
Like all batteries, the amount of energy that the high voltage "propulsion" battery can store will decrease with time and miles driven. Depending on use, the battery may degrade as little as 10% to as much as 40% of capacity over the warranty period. If there are questions pertaining to battery capacity, a dealer service technician could determine if the vehicle is within parameters."