It would be impossible to _actually_ have a feature like this as opt in since it would effectively create two different versions of C#. What the proposal does is emit warnings (and not errors) which is another shame.

I'd much rather have it opt in at the class declaration level like:

`strong class Foo`

Where strong means references to it act like value type references with regards to nullability.

Alternatively, they can just fork C# and remove all the weird parts they've accumulated over the years and stay with a sub-language that's more powerful and they can add useful features on top of.