I’m aware. Algebra is what I’m most interested in, and so when someone says “0” I think “additive identity of a ring” unless context makes the use obvious.
Edit: I’ve given it some thought, and I’m not convinced all algebras can fit in a set, because every non-empty set can have at least one algebra imposed upon them, and so the set of all algebras must have cardinality no less than the proper class of all sets. We also can’t have a set of all algebras (up to isomorphism) because iirc the surreal numbers are an algebra imposed on a structure that itself incorporates a proper class, and is thus incapable of being a set element.