It seems to be broken right now, but https://lemmy.directory/ attempts to be this. The “all” posts feed on your instance shows all posts In communities that someone on your instance subscribes to (which might be quite a lot of communities on a big instance, or the same as your own subscribed list on a single-user instance). lemmy.directory has some script that tries to automate subscribing to everything, so it can be a true /r/all analog.