[jbl]

In the days of friction shifting, it used to be common to mix and match components across the entire drivetrain. You could have shifters, derailleurs, chains, freewheels, and hubs all from different manufacturers.

Drivelines are more integrated now, with derailleurs, cog tooth profiles, chains, and shifters designed as a system with minimal thought given to cross compatibility. At best you could use a non-Campy/non-Shimano chain.