Just to add to Doug’s post, SimBrief works the same way as in the real world for the most part. The calculated OFP is planned with the most likely SID/STAR so that the fuel calculation is as accurate as possible. Almost all real-world airlines, dispatchers, and flight planning systems include a SID/STAR in their OFPs as far as I know.
However, when SimBrief uplinks or exports your latest flight into a variety of third-party add-ons and simulators (Fenix A320, PMDG, etc), the SID/STAR is not imported. Just like in the real world, the pilots need to manually select it from the FMS afterwards.
Also, when prefiling on an online network such as VATSIM or IVAO, SimBrief omits the SID/STAR from the route as needed depending on the country. In North America the SID/STAR is normally kept, while at many European airports it is automatically removed for example.
With all that said, removing the SID/STAR from your OFP is still easy enough to do by simply editing the route text option before generating. I don’t know if or when an option to do this by default will be added as there is not much demand for it.
Best regards,