I think they are, but the players come under pressure to turn them down (think Montpellier).
The Boks have said that they will start enforcing the rule, the clubs can they decide if they still want the players. They then can;t moan as they were warned before they sign the player. The player could retire from International Rugby but for most players the honour of representing your country still means an awful lot and I can't see many (there will be some) refusing to play for their country.
https://www.skysports.com/rugby-uni...a-scrap-30-test-cap-rule-for-overseas-players
"Erasmus also warned overseas clubs he would strictly apply World Rugby's Regulation 9 that guarantees players are available for national team duty 14 weeks of the year.
The Boks have in the past compromised with clubs to allow them to retain players during the Rugby Championship and Autumn Internationals in the northern hemisphere.
"If the clubs don't like that then they have the option of not signing the player," Erasmus said."