Yes, some so-called ‘demo trading contests’ do pay cash prizes to winners.
One thing to keep in mind, however, is that they put these on to encourage new users to join their platform.
I’ve also seen that often these firms run the competitions through offshore entities with little to zero regulatory oversight. What does this mean? Well, it means you may not have many recourse options in case of disputes and their trading operations and processes may not be subject to the same rigour as well-regulated providers.