And based on further feedback, the Post-Format related Guidelines have been changed again. To summarize:
- Post Formats
- Themes may optionally support Post Formats, including any, all, or none of the Post Format types.
- Unless an extension API is implemented in core, Themes are required not to implement a custom extension for Post Format types (e.g. “hacking” the core Post Format type list to add custom types, such as “code”).