After reading Rob's missive above and after studying the new SDK and after testing PILOT's B-314 (as the developer) and other existing third-party addons, I have the following, hopefully useful, additional information for you: most P3Dv5 addons do appear to work with P3Dv6. As regards all of the new PBR stuff; to take advantage of it P3Dv5 SimObjects need to be reworked but it is not necessary to do so to run most existing SimObjects in P3Dv6. There may be a very few SimObjects that do not translate across, but it is most likely that these are doing something beyond the P3Dv5 base sim.
99% of my addons are from Orbx and none of them have thrown a wobbly (yet? 😄)
-Dai (PILOT'S developer / Dragonflight Design)