A glut of newly built high-end condominiums has taken some of the shine off Manhattan’s luxury co-ops, a longtime sign of real estate prestige and exclusivity in New York. Contracts for co-op apartments priced at $4 million or more fell 25 percent this year from 2015, as buyers with means opted for newer homes with more amenities and fewer restrictive rules, according to a report published by luxury brokerage Olshan Realty Inc. It was the biggest annual decline since the firm started tracking luxury co-op contracts a decade ago. “The data right now has a big, red circle on it that says this sector is in trouble,” said Donna Olshan, president of the firm that bears her name. Many buyers see co-ops “as completely outdated and they reject the notion that their equity is tied up at basically the vagaries of a co-op board.” Buyers at cooperatives, usually older properties with fewer amenities, get shares in a corporation that owns the building and don’t hold deeds to their units. The boards can approve or reject buyers and have a say on everything from how much cash purchasers must have in the bank to how much their dogs can weigh. Newly built luxury condominiums — with gyms, pet spas and yoga rooms — are proliferating in Manhattan and giving home shoppers more choices.