As Doug Stevenson pointed out the contents of the GoogleService-Info.plist are public and accessible to every iOS user of your application. Therefore it is inaccurate to say they could be stolen as they're already publicly available.
As explained in the Firebase documentation the file fields contain identifiers used by your application and Firebase servers to route the requests being made to Firestore, Real Time Database and the rest of products the app might be using. Reading the documentation or the post shared by Doug you would see the information exposed is not a security threat.
Moreover, I would like to point out that everybody could try to access your Firestore collections and try to add/drop data. This is indeed expected as Firestore is publicly accessible by mobile and web clients. However, this doesn't mean Firestore is exposed to users to do whatever they please, instead, the actual access is totally under your control by the means of security rules, which enforce what actions a given user could do. In that sense, the developers who left will only have the access level that your security rules grant them, which shouldn't be a threat when having good rules in place.
Lastly, you may revoke the credentials completely removing the application. I'd say it's not a great idea as you would cause a service disruption to app users. Also, this won't improve security or diminish risks.