We can say that google assistant is available for all devices but unofficially, one can use google assistant on any device by customizing the build.prop on the android 7.0 Nougat, you just have to add ro.opa.eligible_device=true at the end of the file.You can also use google assistant on marshmallow but for that you have to install exposed module.But follow this guide to Enable google assistant while installing Open Gapps On Nougat. This Exposed Module is based on feature, introduced in great androidN-ify module and enables google Assistant on Marshmallow ROMs.The option to enable google assistant has been added to Open Gapps which is the google apps package provider and the option will be available while installing the Open Gapps. The devices with custom ROMs can access it and they don’t have to customize the build.prop.
Now there is an Option Enable google assistant while installing Open Gapps:
The option to enable google assistant had been live in the latest build of OpenGapps that was held on 3rd Feb. Google assistant will take some more time to replace google now as there are some features which are still missing. Source
