Any more, it feels like a mandatory to clear whatever it is that we paint, but it was not common in 1:1 until the mid-80s.
Properly applied and gently rubbed single stage is far more preferable to me on a vintage project.
I know there will be waste product flinging, but, a deep clear on a car that didn't have it in the first place is a red flag for me.
Saw a recent HMM cover car at the last Pontiac TriPower nationals, and dang if it wasn't the most spotless of its type I have EVER seen. But... it just didn't connect for me.
Long way of saying, I like the pop a finish gets from clear, but there are limits to every project.