I have a lot of old stuff and when I took the plunge to get Poser 12 in late November 2022 there were no issues for me whatsoever.  You are probably right about backward compatibility.  The textures were in the render but looked slightly darker compare to the P12 render.  I can change what Cycles doesn't like easy enough and hopefully it is only for a few things that I have but I doubt it.  Perhaps some of the shader scripts may have warning errors as well as they are heavy on nodes that Cycles may not like. The only way for anyone to find out is to the Trial Version and see what doesn't work.  :)  Gotta run. I need to be somewhere else.   Have fun or frustration with the new toy... :)