AFAICT, to participate in Stadia at all you have to buy 3 months of a subscription that starts in November. That takes you into 2020.
Re:Stadia Founder's Edition
by CraftThatBlock on Thursday June 06, 2019 @09:50PM
(#20120966)
The founder's edition is just to pre-order the countroller (and Chromecast + few more goodies), but at launch you'll be able to buy the subscription until the free tier becomes available
Until then you will have to buy the game and pay the subscription fee.