Old Reddit is posts from before something like 2009. You now have to login to view Old Reddit.
https://old.reddit.com/login/?reason=lor2&dest=https%3A%2F%2...
It's not that important of a loss in the grand scheme of things. It's probably less than 1% of the posts that have ever been made on the Reddit website, since Reddit was a pretty small and obscure website until 2011 or 2012.
But it might be the first step toward making all of Reddit only readable with an account.