New federal guidelines aimed at clamping down on antitrust violations are likely to prolong a slump in venture-backed startup acquisitions, which have slowed to a crawl amid higher interest rates and unsteady market conditions. 

Venture capitalists, lawyers and analysts say the guidelines, issued last week by the Federal Trade Commission and the Justice Department, are a signal regulators are committed to blocking a wider swath of deals. Though venture-backed companies aren’t specifically targeted, the heightened threat of…