I don't think anybody truly knows the answer, because it will depend on how quickly the various ATS units around Europe get their equipment installed.
And until they have it, they will have no way of telling who has Mode S installed, so even if their law says you must have it, they will never know if you don't.
Here in the UK, the IFR sectors will get it (or already have, in some cases) but a lot of smaller radar units that work OCAS traffic (e.g. certain LARS units) will not get Mode S for years if ever. One LARS unit I visited recently reckons they will never get the equipment.