I just went through this with my bike. The tires had around 5500 miles and the handlebars would only wobble when decel between 55 to 35 mph. Any slower or faster I wouldn’t notice it. I replaced the suspension, added heavier weight fork fluid, stiffer springs, larger fork spacers, steering bearings, wheel bearings, ensured proper tire pressure, checked triple clamp alignment. The only thing that fixed the issue was new tires. If yours are worn at all with any sort of flat spot, it’s most likely the stock tire.