Connecticut, Maine, Massachusetts, and New York

https://www.kff.org/medicare/press-release/in-all-but-four-states-seniors-on-medicare-can-be-denied-a-medigap-policy-due-to-pre-existing-conditions-except-during-specified-windows-of-opportunity/

"This new analysis of federal law and state regulations shows that only Connecticut, Maine, Massachusetts, and New York require Medigap insurers to sell policies to all Medicare beneficiaries ages 65 and older either continuously during the year or for at least one month per year. In all other states and the District of Columbia, insurers may deny a Medigap policy to seniors, except during their initial open enrollment period when they start on Medicare, or when applicants have other specified qualifying events, such as the loss of retiree health coverage."