We are getting a bit afield of the OP's original question, which I think is a moot point anyway. But to wrap it up best I can, I think the option is gone and some other app is necessary. Following any of the suggestions above does not result in reading the code.
1. Now On Tap does not open when I swipe up from Assistant, even with the camera on first.
2. While Google Goggles doesn't crash on me, the camera doesn't seem to work with it. As mentioned, it seems to have been abandoned anyway.
There is a post on product forums which I will not copy the link to since it makes what seems to be a patently false statement about the only way to read these codes being a singular app. From my memory there are several apps dedicated to scanning them.
There are stories that say you can send codes to Assistant, and one that indicated you could tell assistant to "scan a barcode" and it would without any other app. All that did was to suggest an application.
So if scanning a barcode on Android, post Assistant, really takes holding the home button and swiping up, or taking a picture and sending it to Assistant, or telling assistant to "scan a barcode", then it seems you're better off with a separate app, or killing Google Assistant and going back to Now On Tap, which might be better all around since I've yet to find assistant useful--but that may just be the curmudgeon in me.
So that functionality lasted how long? It may be too much to hope for Google settling down and thinking through their software platform before they develop some new shining object, but I hope none the less.