Adobe Captivate Classic is the older generation Adobe Captivate (which was originally released in 2019). It is a legacy tool to create all kinds of engaging and interactive eLearning content.
You do not have to buy Adobe Captivate Classic separately. All subscription plans of Adobe Captivate includes access to both the versions - the all-new Adobe Captivate and Adobe Captivate Classic. You will be able to install, launch and activate both the versions in the same computer.