What problem does this address?

Right now, the process for determining which settings need to be done in theme.json vs the Styles interface is very manual. Tied to this, there's a regularly increasing amount of possibilities coming to each. For example, box shadow support was added to theme.json but it would be great to know when this can be controlled in the UI without needing to manually test or crawl through Gutenberg releases.

What is your proposed solution?

I think there are a few to explore including:

• Adding information about when a support was added so folks can see how things evolve over time/get the latest: Update theme schema docs #43716
• Update the schema to be more verbose/informative around what's valid/supported similar to this effort with block.json: Theme.json schema block properties: valid vs supported #36630
• Create a living document that indicates what's available in theme.json vs in the Styles interface (perhaps by having a "theme.json only" section since we inherently have to start with theme.json first).

cc @WordPress/block-themers for thoughts :)