Expect-CT is an HTTP header that allowed websites to opt in to Certificate Transparency enforcement before it was enforced by default. It also has reporting functionality to help developers discover CT misconfigurations.
Expect-CT was designed to help transition to universal Certificate Transparency (CT) enforcement, by allowing high-value websites to opt in to CT enforcement/reporting for better security before CT enforcement was required (by Chrome) on all public websites. However, Expect-CT has now outlived its usefulness. Chrome requires CT on all public websites now, so there is no security value to Expect-CT anymore. Expect-CT was also designed to help site owners discover CT-related misconfigurations; however, now that CT is universally required, CT is configured in websites' certificates by certificate authorities and virtually never configured by individual site owners, thus Expect-CT has very limited value as a misconfiguration/debugging tool anymore either. No other browser has implemented Expect-CT so removing it is not an interoperability concern.