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There is no clear definition of pixel art. Neither tools, nor technique, not even resolution nor color count define it.

The style of those pics is pixel art. You would not use these backgrounds in a photorealistic or a cartoon setting. You would use them in a game that has a general pixel art style.

Or do you have a better name to describe the style? And to split a hair, something can look a style without being the thing. Like oil painting style for a digitally created image.