Achievements are now live on Stadia, so long as you’re playing via a Chromecast Ultra or on the web. If you’re playing on mobile, they’re not quite ready.
Achievements are one of those features we thought should have launched on day one for Stadia, but this hasn’t been too long of a wait. Important to note, if you did unlock an achievement while they’ve been unavailable, Stadia has been tracking them in the background and you’ll get pop-up windows with all of your unlocked achievements the next time you boot up your game. I can confirm this when I played Borderlands 3 last night.