The classic touring bike was never that popular but had its heyday in the 80s with bikes like the Trek 720, the specialized expedition, the miyata 1000, and a number of other bikes. The formula didn't change much for a long time. Typically touring bikes have a little heavier tubing to deal with the extra weight and the problem of shimmy, a longer wheelbase which made it easier to deal with rear panniers, cantilever brakes to deal with fatter tires, lots of eyelets to help deal with racks, fenders, etc.
That bike is still made (e.g., surly LHT, soma Saga, and Trek 520) but there are some really interesting alternatives out there.
You could go lightweight with your gear and use a lighter and nimbler bike. Or you could get a touring bike designed for off road touring like the Trek 920, REI Mazama, Velo Orange camargue, etc.
Does anyone need a "true" touring bike anymore? If you want to go lightweight with your gear, you don't need a classic touring bike. If you want to carry a lot of weight, why not buy the more versatile off road capable touring bike?