This has never been confirmed I think. It started as idle BC speculation, and then it became BC gospel, but BC are usually full of shit. Fox have 40 years and thousands of issues of FF to work with. One of Marvels brightest stars was assigned to FF for years (Hickman), and the FF took the central stage of the biggest Marvel event since Civil War (Secret Wars). I've always though the reports of Ike Perlmutter killing the FF to spite Fox were a bit silly. I think they're just doing something else with the characters because the main FF book always undersold. Marvel have done alot of screwy stuff with their main characters over the last few years (Wolverine dead, Cap ages, Thor unworthy, Stark a hologram, Hulk dead).