What's a typical implementation of Seq2 in this scenario? (Often a Seq2 is a map, which doesn't have repeated keys.)

PersonAll is not an obvious iterator, because the first element (K=string) is redundant with the second (V=Person), and is not a primary key. Do we expect this kind of thing to be common?

Group is definitely less common than GroupFunc and this example can also be expressed with GroupFunc, but it has its use cases.

Assume we have a people.All() iterator of some kind.

m := make(map[string][]Person)
for p := range people.All() {
    m[p.name] = append(m[p.name], p)
}

Would using a helper function save much here? Sometimes a loop is more clear than a helper function.

For simple use cases like this, I think GroupFunc is more idiomatic. If name was a getter, you would have to use a variable. This conveys its meaning more clearly.

GroupFunc(Values(ps), func(p Person) string {
	if p.Age < 20 {
		return "Youngster"
	}
	return "Adult"
})

Object.groupBy was originally intended to be Array.prototype.group and was only changed due web compatibly issues. So there seems to be a demand for this kind of function. The iterator also supports simple use cases like Limit and Filter. Of course you can say that a loop is idiomatic, but this could be said about every higher order function.