It's a bug report because
versions.txt /
whatsnew.txt doesn't mention anything about its removal. I don't see anything in that file for 7.1 which would explain the removal of this feature (do you even know what it does? :-) )
The closest match I can find is:
Improved interface design for a number of features
Removing borders around buttons isn't really "improving the interface", hence why I'm reporting it as a bug. Maybe the feature got moved into some other portion of the options UI? I don't know.