It missed the launch windows, but Borderlands 3 will be on Stadia within the next month. 2K recently confirmed it on their site, so be looking for it soon.
As explained by 2K, Borderlands 3 is:
At the hard edge of the galaxy lies a group of planets ruthlessly exploited by militarized corporations. Brimming with loot and violence, this is your home—the Borderlands. Now, a crazed cult known as The Children of the Vault has emerged and is spreading like an interstellar plague. Play solo or co-op as one of four unique Vault Hunters, score loads of loot, and save the galaxy from this fanatical threat.
2K has also confirmed that it will be available in 4K:
Play Borderlands 3 on Stadia in stunning high-resolution up to 4K4… without a console. It’s the best way to experience the loot-dropping lunacy of Borderlands 3!
If we learn of an exact release date, we’ll be sure to update things here.