FRANKFURT—The European Central Bank left policy unchanged on Thursday as widely expected, holding fire before a set of crucial decisions in December on ending pandemic emergency stimulus and returning policy to a more normal setting. Confirming its policy stance, the ECB will continue to buy bonds at a “moderately lower” pace this quarter than in the preceding six months, aiming to keep down borrowing costs while the economy rebounds from the pandemic. It also maintained its guidance to keep interest rates exceptionally low for years to come, a stipulation increasingly challenged by financial investors, who are doubting the ECB’s narrative on inflation. The guidance calls for steady or lower rates until inflation rises back to the ECB’s 2 percent target by the middle of its projection horizon and holds there on a durable basis. That is not likely to happen for years to come according to the ECB’s projections, yet …