Why is this change needed?

I see that we are missing the description and it should go here. You can also delete the other two descriptions. But I think we shouldn't have the type and items here since we have the anyOf?

But I think we shouldn't have the type and items here since we have the anyOf?

If we only set anyOf for this schema, we can't update settings in settingsEditor2, which I find inconvenient.
Moreover, I don't think the type string | string[] is a good design. Since we set the default value to [], adding strings to this array is sufficient. The string type seems a bit redundant.
However, to maintain compatibility, we might have to keep anyOf for now.

Only anyOf:

With type and items here:

So, which approach do you think we should adopt?

1. Just remove the type and items fields.
2. Keep type, items, and anyOf.
3. Keep type, items, and anyOf, and add a deprecationMessage for the object in anyOf where the type is string.

// 1. just remove `type` and `items` fields
const choice1 = {
  anyOf: [
        {
            type: 'string',
        },
        {
            type: 'array',
            items: {
                type: 'string'
            }
        }
  ],

  // other properties
};

// 2. keep `type`, `items` **and** `anyOf`
const choice2 = {
    anyOf: [
        {
            type: 'string',
        },
        {
            type: 'array',
            items: {
                type: 'string'
            }
        }
    ],

    type: 'array',
    items: {
        type: 'string',
    },

    // other properties
}

// 3. keep `type`, `items` **and** `anyOf`, and add `deprecationMessage` for object in `anyOf` which `type` is string
const choice3 = {
    anyOf: [
        {
            type: 'string',
            deprecated: true,
            deprecationMessage:  nls.localize('wordSegmenterLocales.deprecateStringType', "Use an array of strings instead."),
        },
        {
            type: 'array',
            items: {
                type: 'string'
            }
        }
    ],

    type: 'array',
    items: {
        type: 'string',
    },

    // other properties
}

2 doesn't really work, because setting the type at the top level seems to just override anyOf. Having this setting work in the settings editor is nice but I'll leave that to @alexdima, whether he wants to change the type.

This shouldn't be 'else if'