I have turned on Content Restriction on my son's iPad. However, there is a bookmark for a restricted site on his Favorites.
I can't Edit Favorites with Content Restriction turned on. (So this is not a duplicate of generic bookmark deletion.)
If I turn off content restriction, I get a different set of Favorites shown, which don't include that one! There is a list of allowed websites in Screen Time > Content Restrictions, but it's not there either.
It's an iPad Mini 2 on iOS 12.1.3. (12.1.4 is shown as an update, but I've not updated yet.)