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)?

A issue menciona Appbar.Content, Appbar.Header, subtitle, subtitleStyle e a especificação vinculada de app bars do Material Design 3. Comece comparando o comportamento da API v5 e as notas de descontinuação com essa especificação; o trabalho estará concluído quando houver uma decisão de um maintainer sobre o comportamento de subtitle compatível ou uma alternativa documentada oficialmente.

Escrita pelo modelo de indexação a partir do texto da issue.