Grid season 3 is still coming to Stadia but will be coming slightly later than other platforms. In a tweet, today Grid responded to a comment asking when the next season Grid is coming to Stadia. They reassured us that it is still coming but like before it will slightly be delayed. Grid is a Pro game and you can experience this for free if you sign up to Stadia at this time.
The Season 3 trailer highlights some of the features coming in this update.