Testing it out I found something interesting. You can put in subreddit.reddit.com and it redirects to reddit.com/r/subreddit. But any 2 char prefix works sends to old.reddit.com (a.reddit.com -> reddit.com/r/a)
e.g. hackernews.reddit.com -> reddit.com/r/hackernews
e.g. hackernews.reddit.com -> reddit.com/r/hackernews
Why does this subreddit exist?