Wo ist Mode S in Europa denn vorgeschrieben?

Europaweit tatsächlich nur für IFR-Flüge. Die Mode-S-Pflicht für VFR ist eine deutsche Regelung.

Notwithstanding possible exemptions and time limited extensions as explained further below, operators shall ensure that by 7 December 2020:

a) All aircraft are equipped with serviceable secondary surveillance radar transponders having the capabilities set out in Part A of Annex II of Regulation (EU) No 1207/2011, requiring these aircraft to be compliant with Mode S Elementary Surveillance.

Applicability
All the above provisions are applicable to all flights operating in accordance with instrument flight rules within the Single European Sky airspace.

https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32020R0587#d1e159-1-1