There’s been a lengthy thread or two about this you might be interested in, for example:
If some forum regulars respond with exasperation, that would probably be why. The threads tend to end up long and somewhat combative.
For my part – I agree that clearing Wanikani isn’t the goal, learning Japanese is. But I’d say that’s an argument for slowing down and learning everything it has to offer rather than trying to push forward and assuming some words won’t be useful!
As far as I know there exists no such tool to add the delete feature to Wanikani currently. With that lack for now, if it helps, maybe think of 造園 as just another small way to practice the readings and meanings of 造 and 園 individually, as you may not come across the specific word but certainly will come across new words with those components.