Define a policy for deprecation markings.

We already deprecate `example` in the Schema Object, but do not
define what deprecations means.  We also effectively deprecate
`allowEmptyValue` in the Parameter Object but did not explictly
say so.  We will likely make other deprecations in the future.

This defines a very basic deprecaton policy (they are still
supported, but we reserve the right to define a policy for
removing them in the future) and marks `allowEmptyValue` with
the same formatting used for `example`.

Author: handrews

src/oas.md

@@ -149,6 +149,14 @@ The OpenAPI Specification is versioned using a `major`.`minor`.`patch` versionin
 
 Occasionally, non-backwards compatible changes may be made in `minor` versions of the OAS where impact is believed to be low relative to the benefit provided.
 
+#### Deprecation
+
+Certain fields or features may be marked **Deprecated**.
+These fields and features remain part of the specification and can be used like any other field or feature.
+However, OpenAPI Description authors should use newer fields and features documented to replace the deprecated ones whenever possible.
+
+At this time, such elements are expected to remain part of the OAS until the next major version, although a future minor version of this specification may define a policy for later removal of deprecated elements.
+
 ### Format
 
 An OpenAPI Document that conforms to the OpenAPI Specification is itself a JSON object, which may be represented either in [[RFC8259|JSON]] or [[YAML|YAML]] format.
@@ -926,7 +934,7 @@ These fields MAY be used with either `content` or `schema`.
 | <a name="parameter-description"></a>description | `string` | A brief description of the parameter. This could contain examples of use. [CommonMark syntax](https://spec.commonmark.org/) MAY be used for rich text representation. |
 | <a name="parameter-required"></a>required | `boolean` | Determines whether this parameter is mandatory. If the [parameter location](#parameter-in) is `"path"`, this field is **REQUIRED** and its value MUST be `true`. Otherwise, the field MAY be included and its default value is `false`. |
 | <a name="parameter-deprecated"></a> deprecated | `boolean` | Specifies that a parameter is deprecated and SHOULD be transitioned out of usage. Default value is `false`. |
-| <a name="parameter-allow-empty-value"></a> allowEmptyValue | `boolean` | If `true`, clients MAY pass a zero-length string value in place of parameters that would otherwise be omitted entirely, which the server SHOULD interpret as the parameter being unused. Default value is `false`. If [`style`](#parameter-style) is used, and if [behavior is _n/a_ (cannot be serialized)](#style-examples), the value of `allowEmptyValue` SHALL be ignored. Interactions between this field and the parameter's [Schema Object](#schema-object) are implementation-defined. This field is valid only for `query` parameters. Use of this field is NOT RECOMMENDED, and it is likely to be removed in a later revision. |
+| <a name="parameter-allow-empty-value"></a> allowEmptyValue | `boolean` | If `true`, clients MAY pass a zero-length string value in place of parameters that would otherwise be omitted entirely, which the server SHOULD interpret as the parameter being unused. Default value is `false`. If [`style`](#parameter-style) is used, and if [behavior is _n/a_ (cannot be serialized)](#style-examples), the value of `allowEmptyValue` SHALL be ignored. Interactions between this field and the parameter's [Schema Object](#schema-object) are implementation-defined. This field is valid only for `query` parameters. <br><br>**Deprecated:** Use of this field is NOT RECOMMENDED, and it is likely to be removed in a later revision. |
 
 This object MAY be extended with [Specification Extensions](#specification-extensions).