Finally able to investigate 19.0, having been in 18.5 until now. The USD Render ROP node in 18.5 had a pretty critical option called Save Style with options to tell Solaris what to do with the various layers on the stage (flatten or not, etc.). I am not seeing that same option on the node in H19. Could someone explain the change? Do we need to implement a separate ConfigureLayer LOP ahead of it now if we want to control layer/stage flattening for rendering?

I'm not seeing it on the usdrender node in the version of 18.5 I have. Are you sure it was there in 18.5?
Why do you would want to flatten, the usdrender works pretty well as is? If you want to control it you can use the usd rop.

Oh I'm a dork, it is indeed on the usd rop, not the usd render rop. (facepalm)