<PackageReference Include="System.Reactive" Version="5.0.0-preview.16" />

EnlightenmentProvider

public static class EnlightenmentProvider
Provides access to the platform enlightenments used by other Rx libraries to improve system performance and runtime efficiency. While Rx can run without platform enlightenments loaded, it's recommended to deploy the System.Reactive.PlatformServices assembly with your application and call EnsureLoaded during application startup to ensure enlightenments are properly loaded.
public static bool EnsureLoaded()

Ensures that the calling assembly has a reference to the System.Reactive.PlatformServices assembly with platform enlightenments. If no reference is made from the user code, it's possible for the build process to drop the deployment of System.Reactive.PlatformServices, preventing its runtime discovery.