Thank you for this PR!
Why is this not reported? I think a, b, and c are Vue component properties and should be sorted.

Thank you for the review.

I have pushed a change that will ensure objects returned by the data() method are sorted.

Just clarifying here, we only want to sort the keys of data() objects and we can leave objects defined within computed, watchers and methods unsorted?

If that is the case then this is all good.

If we want to also sort all new objects defined within a vue instance but not directly attached to the component maybe we should look towards implementing a new option that only sorts keys that are directly attached to the vue instance.

For example

...
computed: {
 // I would like the computed keys to be sorted.
 a () {
 // But I do not want to sort new objects defined within my computed.
 return {
 b: null,
 a: null,
 }
 },
}

Sorry for late reply.

I think this rule should sort the keys of all objects that aren't reserved as Vue component definitions.
Therefore, I think the following b and a should also be sorted. Because they are inside the Vue component.

computed: {
 a () {
 return {
 b: null,
 a: null,
 }
 },
}

If you want to sort only the properties of Vue components (e.g. props, data, computed, watch, and methods), I think we can add another rule to exclude it from being checked by the vue/sort-keys rule.
I think it would be better to have a separate rule for sorting related to Vue components than vue/sort-keys rule, like vue/order-in-components rule.
https://eslint.vuejs.org/rules/order-in-components.html
What do you think?

Hello, sorry for the slow reply, been very busy with work lately.

This rule already accepts an ignoreGrandchildrenOf option that might be applicable here, iirc it only considers grandchildren keys of the same object, not newly created objects. Let me confirm this behaviour when I get a chance and maybe we could look towards making that ignore all grandchildren, not just grandchildren that are directly attached to the grandparent key.

The ignoreGrandchildrenOf option is an option to avoid conflicting with ordering conventions with Vue components.
If we need an option to ignore all grandchildren, I don't think that option should ignore the outside of the Vue component.

I think vue/sort-keys rule should have a role to enforce an order other than the Vue component ordering convention.
I think the order of Vue properties is the ordering convention of Vue components, so I think we need another rule.

I think runOutsideVue=false is just an option to ignore the outside of Vue. (However, I'm not sure if it's necessary for the user.)

I think it should be another rule if you want to enforce the order of Vue properties.
Where do you want to enforce the order?

@ota-meshi I was hoping to enforce order on all props, computed props, methods and watch child keys, eg.

export default {
 // These keys would not be enforced by this rule/option combination.
 name: 'ComponentName',
 props: {
 // These keys order would be enforced by this rule/option combination
 a: { type: String, default: null },
 b: { type: String, default: null },
 },
 
 computed: {
 c () {
 return {
 // These keys would not be enforced by this rule/option combination.
 e: null,
 d: null,
 }
 }
 },
}

I agree, I believe a new rule would be more appropriate for this. I have closed this PR for now and if I get a chance in the future I will submit a new PR, thanks.