You can add the Subi subscription widget to the unpublished or new themes by following these steps:
Go to Shopify admin
Click on the online store on the menu in the sales channel section
Click on the customize button of an unpublished or new theme
Go theme settings
Click on the "App Embeds" tab
Toggle on the Subi Subscriptions app embed
Click on the save button
After finishing these steps, if you've created a subscription plan for a product you must see the widget on it.
Just keep in mind that every change on the customization page of the Subi panel will be applied to both published and unpublished themes.