/quit is not one of DOpusRT.exe's arguments so I am not sure if that command would ever have worked.
If you want to exit Opus from within Opus, the command is in the default toolbars as File > Exit Directory Opus (and a couple of other places). It runs Close PROGRAM=confirm and you can remove the =confirm part if you don't want the confirmation.
If you want to tell Opus to exit from outside of Opus: Exit Directory Opus from a Batch or Script.