and I could swear that 'systray' items were also there in previous versions - but is also no longer there
Probably because many features in the Windows OSs which supported OD 2 and OD 2 Plus have been depreciated. From Redskittlesonly's OP here: https://forums.stardock.com/526582/page/1/#3924920
With the modernization of the application, we did have to sunset some minor features that were tied to legacy components of Windows that have since been deprecated. But the core functionality of the app has been streamlined and with the new support for higher resolution displays,