I'd argue that the Go example is worse than the equivalent in C, because Go has a garbage collector and C does not. In C, precisely because of the lack of a garbage collector, people need to explicitly and carefully document who is responsible for deallocating the memory. This makes things clear who is owning that memory. And this by extension leads to an intuition on whether the append is acceptable on the original slice or not. If a C function returns such a slice and is documented that the caller should free it, you can call realloc and add to it; if the C function documents that the caller should not free it, you naturally wouldn't even try to append to it: you would copy it first.
In Go, this garbage collector frees people from thinking about freeing memory. But it doesn't free people from thinking about owned vs borrowed memory. And that's where bugs can happen.
In Go, this garbage collector frees people from thinking about freeing memory. But it doesn't free people from thinking about owned vs borrowed memory. And that's where bugs can happen.