Hey all,
You can still play the pre-Reforged versions, you just need to have/find copies of them. Once installed, all features will be available except for multiplayer. This is because if you connect to Battlenet, it will require an update to Reforged.
Currently, the easiest method is to grab a copy of the v1.31.1 PTR from the Bliz download site. It’s down under “Classic Games” and is the one named “Public Test Realm”. It’s the final version before Reforged. This option may not be available forever, so I’d grab and install it asap if it’s something you are interested in.
Another method is to find older versions online. Here is a thread in General Discussion that has some good links. Just watch out for files that are full game installers vs patches.
A third option is to install off the original CDs and find a modern patch. The same thread linked above has information on that as well.