append可以分配一个新的数组。它确实可以。在第一个append之后，x现在是一个副本，您正在循环原始数组。对x的更改现在对s没有影响。
让我们用更多的调试来标记这个循环。

x := []string{"A", "B", "C"}

for i, s := range x {
    fmt.Printf("Start: i=%v s=%v x=%p %v\n", i, s, x, x)
    x[i+1] = "M"
    fmt.Printf("After x[i+1] = \"M\": i=%v s=%v x=%p %v\n", i, s, x, x)
    x = append(x, "Z")
    fmt.Printf("After Append: i=%v s=%v x=%p %v\n", i, s, x, x)
    x[i+1] = "Z"
    fmt.Printf("After x[i+1] = \"Z\": i=%v s=%v x=%p %v\n\n", i, s, x, x)
}

And the result ...

Start: i=0 s=A x=0xc0000161b0 [A B C]
After x[i+1] = "M": i=0 s=A x=0xc0000161b0 [A M C]
After Append: i=0 s=A x=0xc000062180 [A M C Z]
After x[i+1] = "Z": i=0 s=A x=0xc000062180 [A Z C Z]

Start: i=1 s=M x=0xc000062180 [A Z C Z]
After x[i+1] = "M": i=1 s=M x=0xc000062180 [A Z M Z]
After Append: i=1 s=M x=0xc000062180 [A Z M Z Z]
After x[i+1] = "Z": i=1 s=M x=0xc000062180 [A Z Z Z Z]

Start: i=2 s=C x=0xc000062180 [A Z Z Z Z]
After x[i+1] = "M": i=2 s=C x=0xc000062180 [A Z Z M Z]
After Append: i=2 s=C x=0xc000062180 [A Z Z M Z Z]
After x[i+1] = "Z": i=2 s=C x=0xc000062180 [A Z Z Z Z Z]

注意在第一个append之后，x的地址是如何改变的。任何x[i+ ] =？no longer affect s`。