ESO is confirmed to have Cross-Play with PC: https://www.forbes.com/sites/davidjagneaux/2019/11/12/elder-scrolls-online-launches-on-google-stadia-in-2020-featuring-cross-play-with-pcmac-players/#744f1...
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ESO is confirmed to have Cross-Play with PC: https://www.forbes.com/sites/davidjagneaux/2019/11/12/elder-scrolls-online-launches-on-google-stadia-in-2020-featuring-cross-play-with-pcmac-players/#744f12e6e592 But yes, otherwise you are correct in that it is up to the Devs. Stadia is fully supporting of any Dev that wants to enable Cross-Play for their titles.