Shares in London-based advertising giant WPP plunged 18% on Wednesday, stoking broader anxiety about the state of the ad business. The stock settled at $29.34, its lowest point since the onset of Covid in 2020, after lackluster quarterly earnings results and the company’s unexpected lowering of profit forecasts. WPP cited a combination of client defections and economic jitters. The sector is facing significant disruption due to AI, which can perform functions like planning, buying and creative execution a ...