It started perhaps a decade ago.
In my opinion there are two reasons why this is now possible. The first is that the center electrode of the spark plugs off of which the arc jumps (on most engines) is now commonly made of irridium. Irridium is some 8X harder than platinum, and erodes much more slowly. Erosion of the electrodes used to be one of the single biggest reason for plug changes.
The second reason is that engines today run far, far cleaner than the engines of old. That means less carbon deposition, better heat distribution, less oil ash, and so forth. Fouling was the second biggest reason for plug changes.
The reasons that engines run much cleaner is twofold, first of all the fuel is far better metered. Electronicall controlled injection in each one of the cylinders, controlled by engine need sensors and a microprocessor, controls the fuel far more precisely, and the far higher pressures that injectors spray at (as opposed to the low pressure output of the orafice in a carburator) creates a much finer spray, for more surface area per volume of fuel. The droplets burn far more completely if they’re smaller.
Secondly, fuel sprayed ring behind the exhaust valve at precisely the right time means that the spray will get combusted at its most finest (pardon the wording). New direct injection engines are even better at this.
So, in summary, better plug materials and cleaner engine operation allow plugs to go far longer without suffering the erosion or the deposition that we used to see.
Having said that, plugs can stick in the hole if left there for 100,000 miles. I ilke to change them out at 60,000 instead.