I am currently creating an app that requires an upload field.
Polaris React has DropZone, but this is not currently available in the new Polaris Web Component, or only under Checkout UI extensions.
Are there plans to add this to the app home and Admin UI extensions?
Yes, the DropZone web component is currently being worked on and should be available soon! No exact timeline, but new Polaris web components will be made available as they are ready.