Not seeing support for the prism legendary. Is this an error with how I'm using the addon, or does shadowpriest rotation not currently support this legendary?

Shadowflame Prism is supported. It has minimal impact on the APL, though.

During voidform, I am never recommended a shadowword death, while mindbender is active, and prism is equipped. Is this correct, or am I possibly not getting the correct information from the addon?

I do receive a recommendation to use shadowword death, prior to using eruption -- just once in voidform, there are no death recommendations.

I am using the version of the addon available via curseforge updates - I will try adjustments on my end, it sounds liek thats what this issue is most likely.

Shadowflame Prism is included in five lines in the APL. One out of the five refers to Shadow Word: Death. That condition is "shadow_word_death,target_if=(target.health.pct<20&spell_targets.mind_sear<4)|(pet.fiend.active&runeforge.shadowflame_prism.equipped)", which means at this point it'll suggest SW:D if the target is under 20% health while fighting less than 4 enemies or if your Mindbender is active while wearing Shadowflame Prism.

I double checked the implementation within HeroRotation. It appears to be correct. Unfortunately, I don't have the Shadowflame Prism legendary to test. I currently have Talbadar and Eternal Call to the Void. However, running a sim with Shadowflame Prism shows very few casts. In the sim, there were eight casts of SW:D, with two being during opening Bloodlust, one during normal combat, and the remaining five with the boss at sub-20%.

As I said, I don't have the legendary (and, frankly, don't play my Shadow Priest very often any longer), but it appears to be working as intended.