Deprecation of arguments and sub-commands

First of all: Kong is a really nice and easy to use tool, and appreciate the work put into it. A big shoutout to all maintainers and contributors ❤️

Now to my question:
As software services evolve, some arguments and/or subcommands may no longer be necessary or not even supported anymore.

Consider the following minimal example:

```go
package main

import (
"fmt"

"github.com/alecthomas/kong"
)

type Config struct {
Foo struct {
A int `name:"a" default:"1"`
_ int `name:"b" default:"1"`
} `embed:"" prefix:"foo."`
}

func main() {
config := new(Config)
_ = kong.Parse(
config,
kong.UsageOnError(),
)
fmt.Println(config)
}
```

In this example I no longer wish to support the `foo.b` option. Calling the application with `foo.b=1337` terminates the application, since the value has no well defined destination to go to.
I know that I could add an annotation `kong:"-"` or use the `IgnoreFields` option prevent this, but as I understand this is intended for non-CLI fields, whereas I wish to explicitly communicate that this previously valid option is no longer supported.
Is there any idiomatic way to express deprecation of formerly valid arguments with `kong` which I may have overlooked, or would this be a new feature?

Thank you already for looking into it!