"Was" like that? .... Some of us still use Objective-C!

You just misinterpreted what NSArray is. It’s not an immutable type, it’s a read-only type. As others already mentioned that’s different things. If you have a read-only variable and exclusive access to it, then you can rely on immutability as well. This is what Rust has, for example. Many languages use this definition as well as it’s much more useful than having some sort of pure immutable type guarantee (which some languages do have with const). Another language that uses this is Kotlin: List is a read-only type. MutableList is a subtype of List. You can get a “const” List by only keeping a reference to the value via a List binding. This can be worked around via casting and reflection, but even in Haskell you can do unsafe things that mutate a immutable value, so I don’t think that undermines the idea.