In this case it is not necessary and I could have use state.status = status, no worries.
state.status = status
But as I'm more concerned with (vuejs.org/v2/guide/reactivity.html) I've got used to always write Vue.set to avoid any pitfalls while developing. So it is just a way to keep the codebase concise (in one mutation it is state.prop = prop and in the other is Vue.set...).
state.prop = prop
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