What problem does this address?

Gutenberg does not provide any way to extend/customize the Global Styles sidebar for things like:

• To display extra help content - text or video. For example maybe an organization has branding guidelines about which colors (styles) are suitable and when they ought to be used.
• Getting other style variations from some source, i.e. ones that are 'generic' and not linked to a theme.
• Some kind of revision log.

Plugins developers interested in such extensions are forced to manipulate the DOM (e.g. injecting custom React components inside an element located with a CSS selector). But this is very fragile, since Gutenberg can change the DOM without further notice, breaking these extensions.

What is your proposed solution?

Add standard extension points to the Global Styles sidebar such as SlotFills, hooks, or a programmatic API, so plugin developers can customize it with a more robust mechanism.

Seems that the editor sidebar is currently extensible via SlotFills (#13357), so I guess the Global Styles sidebar can follow the same approach.