When updating the OnSemble Platform to 6.0 or higher there was a major change in the .RadEditor control that sets the start value of a Portal Text Editor to double spaced. This was done on purpose to follow most desktop editors. The control that we use from Telerik made this change to keep up with modern day browsers according to HTML5 standard specs.
Shift+Enter is the standard way to get the spacing to single as well. This is a standard that has been in most editors for a while. It will always single space the next line.
In framework version 6.0.6 (release notes here) we will be giving you the option to switch to single space in the toolbar of the Portal Text Editor. So the functionality is not totally gone, but the default has changed.
Here are a couple articles here from Telerik to give further details into why the changes were made and what further changes have been made to the Telerik controls.