Alexa users who are Amazon Prime members are reportedly being automatically upgraded to Alexa Plus. Users who have been upgraded can revert to the old Alexa by saying, “Alexa, exit Alexa Plus.” One user claims that they were “flooded with ads” after downgrading back to Alexa.
With Alexa, there will never be an ad-free tier. You’re just paying to have the ads be less obvious/disruptive.