Why is this statement is unreachable?

The loop:

for {
    input = <-ch
    fmt.Printf("%s ", input)
}

never exits (it loops for ever) so anything following it cannot be reached. To make it reachable there needs to be a way for the loop to end i.e.

for {
    input = <-ch
    fmt.Printf("%s ", input)
    if input == "fred" {
        break
    }
}
fmt.Printf("finished") // no longer unreachable

Note: It is possible that this will still loop for ever (if "fred" is never sent on the channel it will not exit). However this cannot generally be determined at compile time.

A common way of looping with channels is to use range; this will exit when the channel is closed:

for input = range ch {
    fmt.Printf("%s ", input)
}
fmt.Printf("finished") // no longer unreachable

and why the func getData finishes without panic when ch gets empty?

It doesn't exit; it's continues to wait for another value to arrive on the channel. However as per the spec:

Program execution begins by initializing the main package and then invoking the function main. When that function invocation returns, the program exits. It does not wait for other (non-main) goroutines to complete.

So when your time.Sleep(1e9) is done and main exits the go routine running getData is terminated.