Yes, subscriptions are backwards compatible and can be used with previous versions of the Product that are still available for download.
Versions released on Nov 2, 2015 and later can be activated with a JetBrains Account username and password or Activation Code.
Versions released before Nov 2, 2015 can be activated with a Legacy License Key.
If you have a perpetual license (purchased before November 2, 2015) for any previous version of the software, you can use it indefinitely with no subscription necessary. However, will you need to update your license and therefore switch to the new subscription-based model to use any newer versions that are not covered by the perpetual license.