The only drawback is that it will only start working after the first piece of content from threads.net has been shared on your instance - for now it returns a 404 not found.
Edit: Mileage may vary, depending on how Threads solves its fediverse integration.
I think there is a misunderstanding in the kbin community.
/d/ blocks a domain, A domain is not an instance.
But i'm not shure what /d/ blocks or not. As far as I know it's what's between the bracket's after a post ()
You're right that it's a bit trickier than I thought. It blocks the shared content: images and other content hosted at lemmy.world can be blocked from https://kbin.social/d/lemmy.world/ , but it will not include links to external sites, nor will it work for text posts. Blocking lemmynsfw.com worked fine for me, but that's of course because it's an image-heavy service.
Whether or not it will work for threads.net off the bat I guess then depends on how Threads interacts with the fediverse; whether it merely shares a link content stored locally, or whether it distributes the content in its entirety.