In short, no, nor was it ever really advisable.
Android Marshmallow did offer "adoptable storage," which formated everything into a single pool, as you put it. It was up to manufacturers whether or not to enable it, and many did not. It also had a number of drawbacks in terms of lost performance and decreased stability. I think I read Google removed adoptable storage in Nougat, but I could be mistaken. If you weren't given the option when you inserted the SD card, then it's unlikely you have the option.