OFFLINE VIDEOS, are they at a risk?
Once you delete any application (in this case, Youtube), you will lose all the data that application has. But with the introduction of iOS 11, Apple added a new feature that would allow you to keep an application's data intact while deleting the application itself. If you have iOS 11, to do this:


Open Settings app and navigate to "General > iPhone Storage"
A list of apps will appear shortly. Choose the application you want to delete.
Press "Offload App". This will delete the app, leave it's icon on the screen, but it will be reinstalled when you try to open the app. Your data is still intact on your phone, but the app is now deleted. I hope this is what you wanted.