Here's a playground for reference:
The broader point is that there is no way in Go to ensure that data passed over a channel will not be modified concurrently without modifying the type of the data specifically for the channel use case.
A deep_copy() primitive would fix this.
With respect to your second point, I see what you mean but I still don't think that's a negative of Go. You can pass in copied values without having to have a deep_copy() primitive (loop map values and copy to new map, dereferencing a pointer, etc.). Like other parts of Go, if you want to make your data immutable there is no syntactic sugar. You have to explicitly write it out.
It is a negative of Go, because Go pushes you into the pit of failure by its design: it is much, much harder to do the right thing (send deep copies of objects over channels) than it is to do the wrong things (send pointers or shallow copies over channels).