Which is considered legacy, system extensions or system kernel extensions?

System kernel extensions are considered legacy as they are older technologies used in macOS that allow developers to extend the operating system's kernel. Historically, kernel extensions provided a way for third-party developers to enhance and modify the core functionalities of the operating system, but they also posed security risks because of their high level of access to system resources and user data.

With advancements in macOS, Apple has moved towards using system extensions instead, which operate outside of the kernel and provide a safer, more secure way to extend functionality without compromising the stability of the system. System extensions are designed to run in user space rather than kernel space, which reduces the risk of system crashes and enhances security.

The other options do not provide the same kind of low-level access and are not considered legacy technologies in the context of extending the macOS system. System services, sandboxed applications, and system frameworks are all components that maintain a higher level of security and stability compared to kernel extensions, reflecting Apple's ongoing focus on system integrity and performance improvements.