Is "dependent flows" a common used scenario in the real world in industry?

Absolutely, to reuse logic in parent flows or to decouple a big flow

I see, would you go as far as saying, for most real world large customers, it would be unusual to not have flows-of-flows in production?

Not necessarily because you can have many flows but not use flow of flows. I think it’s use case dependent. There are situations though that call for flow of flow specifically. For example, what if you want to use the DaskExecutor for certain steps? Use different images for different parts of the Flow? Run part of the flow with more resources? I think these are better achieved by flow of flows