If you don't specify types at all, is that really opting out? This is valid TypeScript code:
function foo(a, b) {
if (a != 3) {
return a + b
} else {
return "hello"
}
}
console.log(foo(2, 5)); //prints 7
console.log(foo(3, 5)); //prints "hello"
You can compile it and run it on the typescript playground:
https://www.typescriptlang.org/play/This is not like C#, where would be required to specify that every type is "dynamic". Here, you can optionally choose to specify that the types are "Any", but it's still gradual typing.
EDIT: you said "implicit opt-out"... which sounds like a synonym for "opt-in". I don't think "implicit opt-out" is a term.
