As usual, you can never be sure unless you read the code, luckily, it's very small: https://github.com/vitorbaptista/shellshare/blob/master/publ...

The only thing sent to shellshare's servers is the text in your bash terminal. There's no return channel for the servers to send commands back to the computer.

You could argue that you're giving control of the terminal because you're running a third-party executable, but that's the same for any executable you run.