The Gothamist has this article by Ross Barkan, pointing out that New York state courts have ruled three times that the state constitution does not allow the state to outlaw fusion, unless the state constitution is changed. The decisions of the highest state court on that subject were in 1910, 1911, and 1973.
A commission in New York has been empowered by the legislature to implement public funding of campaigns for state office, and also to ban fusion. But of course the commission can’t ban fusion if the state constitution guarantees fusion.