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Did I miss something in school? Plenty of things heavier than helium aren’t metal. Boron, carbon, nitrogen, oxygen, fluorine, neon…
In astronomy, nearly everything is hydrogen or helium. Like, over 98% of all matter is H or He. So it's very useful to be able to talk about H, He, and "everything else". They call that everything else "metals".
Some stellar atmosphere models also add "alpha", which provides an extra knob for the abundance of alpha-capture elements. If you need anything more than that, you're doing some niche astrophysics.
Alpha being the elements up to iron?
Elements formed by alpha capture. Since alpha particles have 2 protons, it's generally elements with an even number of protons.
Not for astrophysicists. They call basically everything a "metal". Of course they know it's wrong. But they keep doing it to annoy the chemists, I think.
We also label things as prime to trick mathematicians into thinking a derivative has occurred