Why value here become optional?

Does the DateTimeFormatValue already include a string?

Why value here become optional?

I made it optional simply because it's supported, when the value is undefined, it returns the current date. Try out: new Intl.DateTimeFormat().format() (should return the current date), which should be similar to calling i18n.date().

Let me know if you'd prefer it to not be optional, but note that it would still work in JavaScript, it simply wouldn't type check anymore.

Does the DateTimeFormatValue already include a string?

No, see: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Intl/DateTimeFormat/format#parameters (yet it does support numbers, even though MDN doesn't mention it).

By the way, you can test the changes in the Lingui repository itself just be changing the "target": "es2017" to something more modern in the root tsconfig.json (e.g., "target": "ESNext").

Just to give you a little bit more context: with ESNext, the latest Intl.(DateTime|Number)Format.prototype.format types are the following (omitting all non-relevant definitions):

declare namespace Intl {
    // Defined in `lib.es5.d.ts`:
    interface DateTimeFormat {
        format(date?: Date | number): string;
        // ...
    }

    // Defined in `lib.es2023.intl.d.ts`:
    type StringNumericLiteral = `${number}` | "Infinity" | "-Infinity" | "+Infinity";
    interface NumberFormat {
        format(value: number | bigint | StringNumericLiteral): string;
        // ...
    }
}

I believe Temporal isn't yet supported (this will most likely change in the future), and this PR should make it so that Lingui supports Temporal as soon as the TypeScript ES lib types are updated to support it.

Also, note that the date in DateTimeFormat.prototype.format is optional (as opposed to the NumberFormat.prototype.format value), which is why I changed Lingui's i18n.date's value to be optional. Hope this helps. :)