* Module variable type constraints now support an `optional()` modifier for object attribute types. Optional attributes may be omitted from the variable value, and will be replaced by a default value (or `null` if no default is specified). For example:
```terraform
variable "with_optional_attribute" {
type = object({
a = string # a required attribute
b = optional(string) # an optional attribute
c = optional(number, 127) # an optional attribute with default value
})
}
```
Assigning `{ a = "foo" }` to this variable will result in the value `{ a = "foo", b = null, c = 127 }`.
This functionality was introduced as an experiment in Terraform 0.14. This release removes the experimental `defaults` function. ([#31154](https://github.com/hashicorp/terraform/issues/31154))
*`terraform show -json`: Output changes now include more detail about the unknownness of the planned value. Previously, a planned output would be marked as either fully known or partially unknown, with the `after_unknown` field having value `false` or `true` respectively. Now outputs correctly expose the full structure of unknownness for complex values, allowing consumers of the JSON output format to determine which values in a collection are known only after apply.
Consumers of the JSON output format expecting on the `after_unknown` field to be only `false` or `true` should be updated to support [the change representation](https://www.terraform.io/internals/json-format#change-representation) described in the documentation, and as was already used for resource changes. [GH-31235]
* Made `terraform output` CLI help documentation consistent with web-based documentation ([#29354](https://github.com/hashicorp/terraform/issues/29354))
