Why yes! The little ghosts have indeed been around forever, but it's only in Side Order (and I GUESS Return of the Mammalians. slightly kindof) that they've been canonized and given an explanation as to what they are and how they work. Before this, their diegetic/non-diegetic status was pretty up in the air, and we didn't know that they apparently contain an inkfishes' entire personality.
I can't believe nobody else is talking about how Splatoon: Side Order not only confirms souls are a real thing in that universe, but they're also like, a physical substance? Marina says there's a "structure" that contains the soul. Inkfish have a designated Soul Organ.
Additionally, what I've gathered from Marina's explanation of sanitization is that this soul organ is saturated with memory and identity - literally Ego Juice - which can be siphoned out of the organ like drying a sponge. And then the Ego Juice can be molded into a solid, tangible object, aka mem cakes. What the hell?