This new tool is located in Editorial > Editorial Content as robots.txt. This editorial content area can be used to add additional entries to your existing robots.txt file. Your existing robots.txt file has entries already populated by ePublishing which cannot be edited except by a developer. Any entries that you add in the Editorial Content area will be appended to the default robots.txt content and can be edited or deleted at any time. Please use caution when editing robots.txt.

You can find your robots.txt file by going to https://www.yourdomain.com/robots.txt

The EC for robots.txt looks like this. Add new lines in HTML mode, then Publish, Save, and clear site cache. It may take a few minutes and a hard refresh (ctrl + shift + R) for the new entries to show up in the robots.txt file in your browser. We used a comment line in the below example to illustrate how this works (this line doesn't do anything). 

It should look like this in your robots.txt file with the new line added.