You can install old versions of the game, you just need to have/find copies of them.
Currently, the 1.31.1 PTR, which was the last version before Reforged, is still available for download on the games download site. It’s the one named “Public Test Realm”. It likely won’t be available forever, so if you’re interested, grab it while it’s still there.
If you want something earlier, see this thread here:
Currently, the multiplayer works on the PTR, but it’s not very populated. However, all other older versions will not have multiplayer. Connecting to Battlenet will require an update to Reforged.