Physical segmentation was a method commonly used 5-10 years ago when Ethernet/IP IO devices started hitting the factory floors. It was common to house two Ethernet Bridge modules in your chassis to physically segment the IO traffic from the plant traffic.
You’re right, through the use of managed network switches, IGMP Snooping features, and VLANs this practice is no longer needed or common place. Having two physically disjointed networks in your cell is probably easier to implement (knowledge of networking not really needed), however, more expensive to do so. Now you will need two of everything for each network. Also, it becomes cumbersome to diagnose and manage IO traffic when it is physically segmented like this.
I would say physically segmenting your network like this is dated, and the “not so cheap” and dirty way to do it. A well planned network design is the only way to go!