New extension blocks add support for static extension methods, and static and instance extension properties.
Big C#14 addition I've been wanting since they added extension methods. I think this finally lets us create stateful mixins, and solve some niche use-cases where multiple-inheritance would really be preferable over composition.
Inside Number 9. I don't hear much talk about it, but for me, it's up there with Twilight Zone, Black Mirror, etc. for a series of short stories with twists.