null itself isn’t the billion dollar mistake, it’s that literally every object can be T|null. Undefined or null is actually quite useful, as long as you’re expecting it.
Claiming that a reference cannot be null means that if you later realize it has to be, you’ve got a potentially compat-breaking refactoring to make.
So, nullable-by-default is either a good idea or a bad idea depending on lots of complex reasons.
Unfortunately, there's a lot of cultural momentum in this regard.
That exact syntax (modulo langage divergences) is one of the options for “nullable” in Python: https://docs.python.org/3/library/typing.html#typing.Optiona...
The typechecker would only allow on the union operations which are allowed on both types, anything beyond that would first have to use type checks or assertions in order to split the union.
So, again, what are you talking about?
There are some exceptions to the behaviour you describe, like for instance with C# which for the longest time only allowed value types to be annotated as nullable, and only very recently extended this to reference types, and only as an opt-in feature, and the type checker only throws warnings, etc. That would be a case of a language which is not "null safe" but provides faculties for accomplishing that.