"Postponed" would be a better word. At the time, we were aiming for key parts of the feature to be in C# 13, but the design and implementation are going to take more time. Look for Extension Types in early C# 14 (NET 10) previews. source: https://devblogs.microsoft.com/dotnet/csharp-13-explore-preview-features/
C# 13 focuses on flexibility and performance, with top features like params collections for added flexibility, lock object for improved performance, and partial properties to support generators.