Is conservation of energy only for isolated systems?

I am thinking in the mechanical context.

Everywhere I research (e.g. Wikipedia) the law of conservation of energy is stated only for the special case of an isolated system. I am wondering if conservation of energy holds beyond that special case; it seems that it should. After all, if a property holds only under special cases then the property is not a law.

Reading Feynman's lecture 14 volume I, I understand that if only conservative forces act on an object its total energy remains unchanged. For example, a falling object subject only to gravity has a constant sum of kinetic and potential energies. However, the system consisting of just this object is not an isolated system because it is subject to the external force of gravity. It seems this is an instance of conservation of energy holding outside the special case of an isolated system.

Feynman argues that at the fundamental level all forces are in fact conservative. This implies that at the fundamental level conservation of energy applies to all systems. Is this true? If so, why is conservation of energy continually stated under the special case of an isolated system?

(this site's "energy-conservation" tag says "the amount of energy in a system is constant" implying the system need not be isolated, further confusing me)

Answer

There are different ways of stating conservation of energy and accounting for energy, which can make the issue confusing. One such statement is "the total energy of an isolated system is constant". This is true, and is the simplest way to state conservation of energy. This form of conservation of energy is the earliest taught.

There's another way of stating conservation of energy, "the energy in a region changes by the amount of energy flowing into or out of a region, and energy in adjacent regions changes by the same amount". You could call this local conservation of energy, and is a much stronger statement. It not only tells us that energy is conserved, but it also tells us that energy can't disappear from a region and reappear far away. This is the kind of conservation of energy that Feynman is considering, so he can apply it to systems that aren't isolated.