I also listed both Stadia and Cloud Rendering as unsupported because no code is given by Unity but it would probably look very close to the one for the Switch test, baring the respective swap for the two other platforms.
I also let them in the Enum because if they're valid, or validated sometime in the future, it will not break the action's enumeration indexing nor screw with users' work; on the coding side, a mere update in the script will be needed by changing the Enum entries to make them look like all other fully supported platforms, ex: UNITY_STADIA_Unsupported changed to UNITY_STADIA, with the necessary change further down in the script too for the detection phase.
Now, both IsOnPlatformEvent and RuntimeApplication are very similar actions since IsOnPlatformEvent only works at runtime too in Playmaker, even for tests... but because of Unity's two ways of checking for all the available platforms, there are few differences that exist as we can see with the three cases above.
If we let the RuntimePlatform instructions inside the IsOnPlatformEvent script, the user will never see the difference though but it's certainly not strictly conforming to Platform directives.