[SalesForce] LWC: Increased security for immutable @api properties but not documented in winter 20

I noticed that the security for immutable @api properties is increased but unable to find any documentation.

Consider below LWC component:

HTML:

    <lightning-button variant="brand" onclick={sortFruits} label="sortFruits"></lightning-button>
    <template for:each={fruits} for:item="ft" for:index="ind">
        <div key={ft}>
            {ind}: {ft}
        </div>
    </template>

JS:

@api fruits;
sortFruits() {
    this.fruits.sort();
}

We should pass the array fruits from parent component like below Aura parent component:

<aura:attribute name="fruits" type="List" default="['Banana', 'Orange', 'Apple', 'Mango']" />
<c:poc fruits="{!v.fruits}" />

When you click on button sortFruits, it will throw error

Invalid mutation: Cannot set "0" on "Banana,Orange,Apple,Mango". "Banana,Orange,Apple,Mango" is read-only. at ReadOnlyHandler.set

Solution:
This can be fixed using this.fruits = [...this.fruits].sort();.

Question:
1. This started throwing error only in winter 20 upgrade and not before. Ofcourse, it is logical error but I do not find any documentation regarding this change so that our developers can do required changes in existing LWC components – not just for sort() but also for any other potential JS functions where code can break
2. When I say this.fruits =, I am actually modifying the data which came from parent. This is working today, but will this work in future? And is reassigning not same as mutating data – in what way they are different?