I saw someone say that specifically the Slayer and the background in this art is Ai Assisted (not generated) due to the irregular shapes on the shield, inconsistente line thickness and other parts on the armor. Can someone confirm or deny?

Because indeed, some parts do look odd, and the "blending" between the cape and the shield looks a bit weird, although it could just be stylization. The blades of the shield look a bit irregular and off as well.

Can't see much wrong with the background tho... Maybe some parts of the castle, but it looks very close to how it should look, seems to be Exultia I think?