I don't have a Pro1x, and it has been so long since I had stock, I can't say for stock. But, yes, with Lineage OS on the Pro 1, you can do that. Under settings>display>rotation settings, you can actually allow or exclude all or any of 0, 90, 180 or 270 degrees.
I don't use autorotate at all, just using the built in ability of the Pro1 to change from landscape to portrait by opening and closing the keyboard. In addition, there's a clever little lock-unlock icon that pops up that can be used in special cases. When I open Kindle, I leave the keyboard out and turn the screen so it is