Description
Generic types for FormTypeInterface and the various createForm methods have been rejected in Symfony because they are lying about the actual behavior of the form component. Those generic types allow to have a nice inferred type for the basic usage of the component, but they will analyze advanced usages of the component in a broken way (reporting errors on valid code, inferring wrong types due to making wrong assumptions about how the component actually works).
See symfony/symfony#40783 (comment) for the discussion when those generic types were submitted upstream.
In version 1.x of the extension, those generic types were configured in skipCheckGenericClasses, which allowed projects to mostly ignore the fact that the extension stubs were adding it. In 2.0x, this is not the case anymore so phpstan will enforce that any form type defines such generic type to satisfy the extension, even when being aware that those generic types are a lie.
My opinion is that those stubs should be removed to match the decision done in Symfony. However, I would also accept a case where the loading of those simplistic generic types is conditional, allowing projects to decide whether they want them (if they rely only on the basic usage pattern where the inference would produce a good result) or not (if they want the analysis to stick to the true behavior of the component).