You can now exclude app category placements on the display network and YouTube in smart shopping campaigns. However, you can not exclude websites, youtube channels, or youtube videos, and you can not exclude apps. Only app categories.
In Google Ads, click on your smart shopping campaign > content > click on the edit icon. Now a popup shows to exclude from the Display and YouTube network.
It’s not yet available in performance max campaigns, which is surprising, considering smart shopping will sunset in 2022, replaced by the new performance max campaign.