When inspecting the interface results, there is an option to see the Plastic Interface Points via the Stresses menu. However, in PLAXIS 2D 2018.00, when selecting this option, Plaxis Output responds with a bug report window, after which the only option is to restart the Plaxis Output program.
When inspecting the bugreport, this information is included in the general section
exception class : EDataProviderResultNotAvailable
exception message : Data-query failed for ctPlasticPoint
In PLAXIS 2D 2018.01, we can now see Plastic Points in interfaces again!
As a workaround in PLAXIS 2D 2018.00, you can inspect the plastic points from the main 2D Soil plot. Fortunately, in this plot, the interface plastic points are also shown here, so you can still see them here mixed with the 'normal' soil plastic points.