Currently, there is a not a good resource that outlines exactly what "block" features you can use within classic themes. For example, you can use theme.json, various add_theme_support() opt-ins, block-based template parts, and more.

The proposal is to give an overview of the essential pieces of creating a hybrid theme (i.e., a classic theme that opts into various block features).

This is a middle ground between classic and block that we haven't really embraced much, but it is still a common use case for agencies and developers who still need to use classic things for their projects.