> The only thing in your example that does things in-place (and thus looks out-of-place)

That is not really correct, and is much of the issue: Compact and Delete operate in-place on the backing buffer while copying the slice metadata. This is the same issue as the append() builtin and why it's so fraught (before you get in all the liveness stuff).

> s := slices.Compact(s) // s already exists, this is not valid Go syntax.

    s := []int{}
    if true {
        s := slices.Compact(s)
        _ = s
    }