Following an announcement late last year that the app would be discontinued, Microsoft has removed the Authenticator app from the Apple Watch App Store and also from Apple Watch devices in an update released today.
The companion app, which was launched in 2018, allowed users to approve login requests directly from their Apple Watch without having to open their connected iPhones.
Unfortunately, in the latest update for the app, it removes it from the Apple Watch entirely. You can still mirror notifications from your iPhone to your Apple Watch to get notified of approval requests.
On the Authenticator FAQ page, Microsoft has the following to say:
Q: Is Apple Watch supported for Authenticator on iOS?
A: In the upcoming Authenticator release in January 2023 for iOS, there will be no companion app for watchOS due to it being incompatible with Authenticator security features. This means you won"t be able to install or use Authenticator on Apple Watch. We therefore recommend that you delete Authenticator from your Apple Watch. This change only impacts Apple Watch, so you will still be able to use Authenticator on your other devices.
You can download Microsoft Authenticator from the App Store, Play Store, and Chrome Web Store.