Here’s another take on this age-old discussion and debate. I think the debate is more closely related to the question, “If a tree falls in the forest and no one is there to hear it… .does it make a sound?”
Seriously though, IMHO, sw products, cameras, techniques, etc. can’t be ethical or unethical. People are ethical or unethical. The use of a tool isn’t unethical - at least that is until the person using it makes a conscious decision to deceive. The the sky replacing example. If you replace a sky - or anything really and then go about presenting the final image as if it happened exactly as you presented it. That’s unethical. If, on the other hand, you disclose and explain - you’re not deceiving anyone. @John_Williams post is a perfect example. Multiple exposures for focus stack, exposure in a fleeting light situation. Did he replace/alter the sky? Yup, but he was totally upfront about his approach and processing.
Another way I look at it. If you’re selling wildlife nature prints under the name of “Tom’s Wildlife and Nature Prints” AND you have prints in your portfolio where bugling elk from Yellowstone were composited with a foggy scene in the Redwood forest of California AND you were selling under the guise that it really happened. That’s unethical IMHO. IF, on the other hand, you’re selling your work under the name of “Tom’s Creative Wildlife and Nature Photography” and you have images of a 500mm moon over a 35mm landscape and calling it creative art… I’m good with that too. Your audience get that you are “creating” something that probably didn’t happen for real.
Are there a thousand other scenarios that we all could debate? Yup - cloning a beer can or twig… black & white? B&W - The ultimate alteration of reality - yet, for that, it’s pretty much established, b&w is what it is and everyone knows it.
I think many of us struggle with this in general - especially us old timers that came from shooting for decades with film. You didn’t get it right in the camera, in the field - too bad. Go read @Alister_Benn 's AMA as he touches on this.
I struggle because I’ve taken advantage of the digital age and “manipulate” my images, unlike I ever could with slides. Although honestly, we used to paint toner on slides to reduce hot spots; mask/crop slides with silver tape, etc. etc… so “manipulation” has been going on since the beginning.
Anyway, to sum it up, again, it’s people that are ethical or unethical. You as a photographer attempt to deceive viewers claiming something is real or happened in one frame, but actually didn’t - you are being deceitful and unethical. JMHO