If your Adobe Captivate subscription began when the previous version was current you would be eligible to use the previous version. For example, If you began your subscription this time last year (November 2016), you would have begun your subscription when Captivate 9 was current. Even if you are currently running Captivate 2017 under the same subscription you would be eligible to continue to use Captivate 9 even today.
Since this is essentally a user forum, I would like to have a staff member to answer your question. AFAIK downgrading was not that frequently available, and you often had to pay for it like when wanting to downgrade Win10 to Win7. I will try to contact someone to give you an official Adobe answer.
Please check the PM, I have sent.
I just chatted with a rep. This is the reply I received (for those still looking for an answer):
Adobe Captivate 9 is no longer available for purchase.
However you can purchase Adobe Captivate 2017 volume license from your nearest Adobe reseller and can provide us the serial key and order number and then we can provide you the Captivate 9 serial key by doing backward licensing.