As the others have said, even Ford continued to use their fender mounted relay (it's a relay, never was a solenoid and Ford always called it a "starter motor relay" in the diagrams) even after changing to the new PMGR (permanent magnet, gear reduction) starters with the piggybacked solenoid (which actually IS a solenoid). Primarily for two reasons that I can see.
Isolation of electrical signals and wiring convenience.
Most older GM vehicles that used the starter solenoid's large battery post as their common termination point also had heat and contamination issues after a few years of use. And to access any of those wires you had to get underneath or reach way down into the engine compartment.
Ford changed the relay's basic layout, but left it on the fender where it stayed away from most heat, and remained a very easy to use "power point" to connect a lot of wiring.
Oh, and it uses less current to energize than a solenoid does too, so continues to take some of the load off of the dash mounted ignition switch. Almost forgot about that part.
The "isolation" aspect is that the starter motor turns into a generator as it's spinning down, and can energize the piggyback solenoid momentarily. This causes the starter motor to run-on for a few seconds after starting.
Probably not good, but at the very least it freaks everybody out when they've just done all this re-wiring work only to have the starter stick on!
I'm not really sure what was different about all the years of GM starters that did not run-on after the key was released, but it happened way too often after swapping in the new starters on our Fords, so it was deemed best to leave the relay in place.
Just like Ford did.
Paul