I suspect the answer will be found in the commodities or intermediate products industries. Companies that only make paper clips, or pencil leads, or a certain type of fastener, or toothpicks, that sort of thing.
I’m sure there are lots of software companies that might qualify. Do the people who make Slack make anything else? What about Snapchat or Instagram before being purchased. There are also thousands of smaller software firms who have built themselves up around a single vertical market like process control, supply chain management for a specific industry, or an imaging package for medical use, or whatever.
When you get into the highly technical vertical markets, there is all kinds of specialization, like a company that might make nothing but drill bits for oil platforms, or companies that provide a specialized chemical for some industrial process.
This thread is biased towards consumer goods, probably because that’s the first thing to come to mind for most people. But consumer goods companies are probably the least likely to have a single product because once you have built the production, marketing, sales, and support processes for your product, adding a second is much less costly. So economies of scale start to drive you towards diversifying your product line.