A quick guide on how to hide upsell items from your main product catalog:
1/ Duplicate or create the product that will serve as an upsell only.
2/ In the product configurator, go to Product Status > Sales Channels and Apps > Manage
Deselect all channels, but the Shopify GraphiQL App
If you don’t see this channel, you can add it by installing the Shopify GraphiQL App.
Go to the install link > Select the scopes:
3/ Any app that has API permissions will be able to pull all info on this product. Even when you add inventory, shoppers won’t see it on the store front end.
If you're using an app that doesn't pull in this new product in the resource picker, contact their support team. They should be able to set it up. Or try out UpsellPlus! We have this functionality default enabled in our app.