Can I backup my whatsapp data from an iOS 8 device and use it on an iOS 7 Device?
Conventionally there is no way to do this between the devices but you can access WhatsApp chat logs via many of the various "iCloud Recovery Tools" out there. Syncios is the one that I use but it's around $50 (trial might do it for free) for a subscription but it does allow you to export those chat logs (I think to an XML off the top of my head). They're a dime a dozen those programs so you'll find one that suits you if it's the route you wish to take. They will require that Two Factor Authentication is disabled.

Sure you won't have them available on the device but you will in the short term have a copy of them for reference.