This was introduced in Build 242:
This release also contains a change to our default stereo/mono playback behavior when talking to multi-channel devices. Previously, the full channel layout was always sent, with silence sent to unused channels. Now, stereo/mono content is sent using the “2.0” layout, and only multi-channel content uses the selected layout. There is a new setting that allows users to revert to the old behavior if desired, but we expect that the new default will work for the great majority of use cases.
I’m able to do the stereo -> multichannel upmixing automatically on my AVR (a Denon 3080), so when I send a pure stereo stream, the Denon upmixes this to what Denon calls “7 channel stereo”, and when Roon sends multichannel, the Denon automatically turns off the upmixing and plays it straight.