True, and the supply is dwindling.
When warbirds were cheap and easily available, most of the planes running at Reno were acquired and modified for racing. I seriously doubt anyone has cut up a good airframe in more than 25 years or even longer. But is you are assembling one from parts of multiple planes, then it isn't such a crime if some mods are done.
Steve Hinton's group is restoring an F-86 at Fighter Rebuilders now. It started life as a Canadair Sabre Mk 6, but it is being modified with slats on the leading edges (not a crime in any pilot's book), the "baggage compartment" is being modified for better storage, and some items that are not needed will be unused and given to the new owner for disposition as he sees fit. From the outside, the only thing that could be noticed are the slats. Nobody not an expert at Sabres would ever notice. But when they complete the wing, I doubt it will be able to expend standard ordnance.
For really detailed restoration, Paul Allen's collection is second to none! The P-40 even has modern wiring with period-correct insulation on the outside of it, complete with stock wire numbers and colors! His Il-2 is stock except for a left-turning Allison V-1710 in place of the Mikulin and a Curtiss Electric prop in place of the Russian prop. The rest is bone stock, but really much nicer than stock. Real Il-2's were not well-finished. Paul's IS.
Carl Scholl at Chino (Aerotrader) is doing a restoration of a B-26 Invader complete with working gunner position (the gunner can select the upper or lower turret and operate either one)!
So a really good restoration depends almost entirely on the goal of the restoration. If the goal is 100% combat stock configuration that is quite different from a clean, well-done bird that can be flown, but without many of the military items such as armor plate, guns, operating wing ordnance racks, etc. Some guys even want the period-correct radios, though it's hard to imagine why. I've seen a 2-seat Hawker Hunter with glass panel and larger engine up in Idaho ... it's awe-inspiring, but far from stock.
If you have something like a Grumman S2F, it probably isn't even legal to restore it as flown in service because it could jam a city with the aerial antennas!
So ... to be really accurate, the term "restoration" must be defined with regard to the project you are speaking about specifically.