I think Aether comes close: https://aether.app/features/general/

There are reddit-likes on the fediverse (which means they are using the ActivityPub protocol). The fact that a software like kbin works on ActivityPub means that users on other fediverse software like Mastodon (twitter-like) can interact with posts on kbin. Discussion, interaction and discoverability are thus not limited to just the small community on kbin. https://kbin.social/

Here is a discussion by @feditips@mstdn.social on reddit alternatives on the fediverse. https://mstdn.social/@feditips/110476830253102884