Beginning in C# 12, types, methods, and assemblies can be marked with the System.Diagnostics.CodeAnalysis.ExperimentalAttribute to indicate an experimental feature. The compiler issues a warning if you access a method or type annotated with the ExperimentalAttribute.

The Windows Foundation Metadata libraries use the Windows.Foundation.Metadata.ExperimentalAttribute, which predates C# 12.

See https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/attributes/general#experimental-attributes