That's basically what c# has done. But it's implemented as a warning which can be upgraded to an error. I think it might even be an error by default in new projects now.

Holy shit, how didn't I know they'd taken it this far? This is great! https://learn.microsoft.com/en-us/dotnet/csharp/nullable-ref...

They actually fixed the billion dollar mistake...

They didn't. A proper fix would require getting rid of null altogether in favor of ADTs or something similar. I work with C# daily and nulls can still slip through the cracks, although it's definitely better than @NotNull and friends.

I haven't worked with Kotlin in a while, but IIRC their non-nullable references actually do include runtime checks, so you cannot simply assign null to a non-nullable and have it pass at runtime like you can (easily) do in C#.

they won’t change the default reference type to non null. might take a few years but you can see their planned syntax here: https://openjdk.org/jeps/401