Notably, Digg updated which also involved a worse interface and didn’t have an “old Reddit” interface you could access. Going to a site that was like the old interface involved leaving Digg and joining Reddit.
That is likely why you can now access older Reddit interfaces. They feel that many people will stay if they can find a way to use the new interface (and they may be right about that). The Digg approach of forcing all to use the new interface was a step over the line for Digg and Reddit likely fears a similar thing could happen to them.
I gave away something on Reddit recently and found out that DMs aren’t the same between new and old Reddit. I got a reply on old Reddit and thought was the only interested person and latter logging in on a different browser found more messages that I didn’t see on old. The messages from old didn’t show up on the new site as well. What a mess.