The problem here is that a court has determined that the referendum is illegal. So helping to do an illegal act is not allowed, and preventing it is one of the

The problem here is that a court has determined that the referendum is illegal. So helping to do an illegal act is not allowed, and preventing it is one of the exceptions of free speech. I think advertising for drug dealers in the USA also wouldn't be covered by free speech. The other problem is using public funds for something illegal. The use of public money is very restricted in Spain. There is a body of civil servants (interventores) whose only job is approving and supervising every euro spent by any gobernment entity. If there is public spending involved in the development or promotion of this app this is a big problem.