Yea, not VList exactly, but similar and also pioneered by Phil Bagwell. They're implemented as a trie under the hood. And provide a list or set or stack or queue or map like interface. You can not mutate the value at any node, instead edits are diffs in the tree, a bit like git commits.

So they end up being almost as fast as their normal mutable counterparts, and with similar and sometimes better memory consumption, because nothing is ever copied under the hood.

This allows them to be good enough to be made the default datastructures. Clojure was the first to make this gamble, and show that the state of the art immutable datastructures mostly pioneered by Bagwell were practical and could replace immutable ones for the 99% use cases.

No: the idea that immutable is a sensible default, and active enforcing of immutability. In Java, e.g., a linked list's cell might be immutable, but its contents are most likely mutable - so back to page one. It is definitely possible to write code for structures that are immutable by interface or convention, but the environment does not help you doing so.