I don't recall accusing you of being totally ignorant or knowing nothing.
That's sure how that "welcome to the club" line sounded. It was needlessly dismissive.
I'm just trying to separate speculation and theory--however well informed and otherwise--from actual knowledge. I enjoy trying to figure out how things are done, but no matter what I come up with, I wasn't there looking over the shoulders of the artists watching all their techniques.
But we can narrow down the possibilities. I'm certain that the auras in that screencap are out of focus rather than airbrushed or backlit; all three look different, and that is definitely a focus effect. And there are only two possible ways that two images in the same frame of an animated cartoon could be differently focused. One is if they're double-exposed, and the other is if a multiplane camera
was used. So we can say with confidence that it was done in one of those two ways. And I'm not sure Filmation had a multiplane camera at the time. I don't recall TAS doing any focus tricks to give a 3D appearance, with foreground and background objects being in different focus. And from the transparency of the yellow color in various other screencaps, I'd say it's far more likely a double exposure than a multiplane overlay.
However, in this screencap from the same episode, Uhura's force field aura definitely looks airbrushed instead:
You can see that the edge of it has a subtly speckled effect, a telltale of airbrushing which is not present in the earlier screencap.
Looking over the force field belt shots in other episodes, like "Beyond the Farthest Star," shows clearly that the look of the auras differed from shot to shot. So I think we can reliably conclude that they used more than one technique. Some shots did indeed use airbrushing, others probably used double exposure. So some could've used some kind of multiplane effect as well, though that's iffy. As is usually the case in animation and special effects, they used whatever technique worked best for each shot, rather than relying on only a single technique throughout.