Junos OS is the FreeBSD-based operating system used in Juniper Networks routing, switching and security devices. Starting in Junos OS Release 16.1, It did not maintain the SDK programming function. But alternative feature provides a rich set of APIs to program the Junos OS control plane. JET allows users to build applications on top of Junos OS and hence, replaces the legacy Junos SDK.
From security point of view, it is a good decision. Although the new approach will be reduced potential risk given by custom SDK development. However it is hard to avoid vulnerability occurs in product itself!
For more details, please see below details for reference.