Subtitle: Why was the subtitle prop deprecated in Appbar.Content when MD3 explicitly supports subtitles in app bars?

The subtitle and subtitleStyle props on Appbar.Content are marked as deprecated in v5.x, yet the [[Material Design 3 specification for app bars]](https://m3.material.io/components/app-bars/specs) clearly shows subtitles as a supported and documented part of the component across all app bar sizes (small, medium, and large).
As seen in the MD3 spec, all center-aligned and expanding app bar variants include a Headline + Subtitle layout.

**A few questions:**
- What was the reasoning behind deprecating the subtitle prop in the v5 / MD3 theme update?
- Is there an officially recommended alternative for rendering a subtitle under the title in Appbar.Header?
- Are there any plans to reintroduce subtitle support, perhaps tied to the mode prop (e.g., only for medium or large modes where the spec shows it)?

Das Issue nennt Appbar.Content, Appbar.Header, subtitle, subtitleStyle und die verlinkte Spezifikation für App-Bars von Material Design 3. Beginne damit, das API-Verhalten und die Hinweise zur Veraltung in v5 mit dieser Spezifikation zu vergleichen; als abgeschlossen gilt die Aufgabe, wenn eine Maintainer-Entscheidung zum unterstützten Verhalten von Untertiteln oder eine offiziell dokumentierte Alternative vorliegt.

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