We are about to develop a notification solution using Google FCM. The customer deploys Android mobile to the users which are highly guarded, and they require that the use of our solution does not enforce them to create Google accounts. Will that be easy to promise ?
Using Firebase requires that you as the developer have a Google account.
But your app's users don't need to have a Google account, unless you want them to.
In fact, if you don't use Firebase Authentication, you can work with completely unidentified users for most services. Firebase Cloud Messaging for example doesn't require users to be signed in.
But you could use Firebase Authentication's anonymous authentication, to have anonymous-but-identified users. With anonymous authentication you don't know who the user is, but you can still know if the same user is returning.