I had a bit of a faff last year trying to clarify the situation with Belgium with their own authorities! In the end the conclusion was - as applied to this question here - yes: you will need Mode S if you fly in airspace currently requiring SSR. More simply put, if you intend to fly in a Transponder Mandatory Zone (TMZ) you will need Mode S. The answer is to avoid the TMZs, providing that is an option in destination/routeing terms, and continue flying without Mode S. Standing by to be shot down by more rule changes which have been buried away by another Himalayan-sized range of mountains....!