Static Site with `Free` sku doesn't support `identity` anymore?

A `PUT` request against the static site endpoint (at least for 2019-12-01-preview and 2021-02-01) with below payload fails recently:

```json
{
"identity": {
"type": "None"
},
"location": "West Europe",
"tags": {},
"properties": {
},
"sku": {
"name": "Free",
"tier": "Free"
}
}
```

With error:

```
{
"Code": "BadRequest",
"Message": "SkuCode 'SKU' is invalid.",
"Target": null,
"Details": [
{
"Message": "SkuCode 'SKU' is invalid."
},
{
"Code": "BadRequest"
},
{
"ErrorEntity": {
"ExtendedCode": "55925",
"MessageTemplate": "SkuCode '{0}' is invalid.",
"Parameters": [
"SKU",
"Free"
],
"Code": "BadRequest",
"Message": "SkuCode 'SKU' is invalid."
}
}
],
"Innererror": null
}
```

While if change the sku to `Standard`, or remove the `identity` block, it works. Did the service team remove the support of `identity` from the `Free` SKU?

Start with the static site PUT endpoint and compare the 2019-12-01-preview and 2021-02-01 payloads shown here, especially the identity block and Free SKU. Check the relevant Azure REST specification and service behavior to determine whether this combination is supported; done means the support status and any required specification change are clearly established.

Written by the indexing model from the issue text.