Unfortunately, that is the case. You cannot subclass a Swift class (even if it is a subclass of NSObject and available to the Objective-C runtime) because of deliberate limitations baked into Obj-C to block subclassing Swift classes in Obj-C code.
I believe the reason for this limitation is that Swift includes features that cannot be utilised in Obj-C and therefore subclasses would be restricted and would get undefined behaviour when implementing methods that cannot cross into Obj-C.
If Apple were to allow Obj-C -> Swift -> Obj-C subclassing, then it would be on a limited basis. Some methods wouldn’t do the same thing in Swift as they would in Obj-C selectors, and you could theoretically declare conflicting methods in your subclass that would have different actions depending on whether you were addressing the class in Swift and Obj-C. Additionally, the Swift Compiler couldn’t see beyond the Swift barrier and therefore may make optimisations that would break your Obj-C subclasses.
While I understand the frustration behind this, both in personal projects and philosophically, I think that this is unfortunately the better of the two options.
摘自：https://forums.swift.org/t/cannot-subclass-a-class-with-objc-subclassing-restricted-attribute/5674
结论：OC里无法继承Swift类，即使Swift是@objc open 修饰也不行。（2021.07.18 记录）