[notnullorvoid]

Some libraries that feature signals style reactivity do use getter and setters (ember.js, and mobx are 2 good examples). However it makes sense for the primitive API to use functions since getter and setters are functions under the hood and get applied to an object as part of a property descriptor. It's also not always desirable to have a reactive value embedded in an object, sometimes you just want to pass around a single changeable value.

As for why some libraries choose the `[thing, setThing] = signal()` API (like solid.js) that's often referred to as read write segregation. Which essentially encourages the practice of passing read only values by default, and opting in to allowing consumer writes on a value by explicitly passing it's setter function. This is something that was popularized by React hooks.

Either way this proposal isn't limiting the API choice of libraries since you can create whatever kind of wrappers around the primitive that you want.