This function in DTM when called in the context of a link click event would return true or false based on the href of the link (if any), and whether the domain of the href was within the property's configured domain list.
This function was used in the custom condition of the DTM out -of-the-box event rule, "Outbound Link", so it's a common migration issue.
There is no function in Launch that provides the same functionality. There is no extension (yet) that provides an outbound link condition either. At this point, the best that can be done is to create a Core > Custom Code Data Element which will return an array of Domains that are considered "Internal" (i.e. not outbound). Then create another Core > Custom Code Data Element (call it 'isOutBoundLink') which will evaluate event.detail.href and return true if its domain is not within the list of internal domains.