Opus has never had a built-in DXF viewer, so that must have been coming from something else you had installed.
If it was something that installed a Preview Handler, then it would work in Opus (and File Explorer, and Outlook) if whatever it was was installed again. (Or it may need repairing if something took over the registry settings involved.)
On the other hand, it may have been a viewer designed for ActiveX or Internet Explorer, which would require some additional setup to make work in the viewer pane (as well as installing whatever provides the viewer itself). An example of setting that up for the Opus viewer pane is described here: