Authenticator app for iPhone?

If you're referring to sites or services that require 2 factor authentication, the answer is yes... and no...

It can be done, but it's not exactly convenient.

I find it easier to use an app like Microsoft Authenticator. I'm up to 18 different accounts / services set up in MS Authenticator now...

All of the apps that I currently have use Face ID (or Touch ID) for authentication, but it is possible that there are still some apps that use an authenticator app. It is up to each app developer to decide what to support.