With the newest version of XSplit Broadcaster, I think it started since 3.7? You can only use Steam Only on Windows 10 OS.
But, stream only can still be used on other Windows OS like 7 and 8/8.1 using an exe parameter.
What you need to is right click on your XSplit Broadcaster shortcut, then go to Properties and at the end of the File path add this parameter enableusestreamonly:1 make sure there is space in between.
Should look something this like this:
Make sure that XSplit Broadcaster is closed when doing this!
Also, take note that any issue you encountered using stream only on Windows 7 and 8/8.1 will not be fixed since XSplit does not support it on this OS anymore.
Let me know here if this works for you or need further help on how.