As of a few months ago, Netflix abandoned in-app purchases entirely on Apple platforms. Try it yourself: sign out of your Netflix account on iOS and try and create a new account. There is no option to pay for a Netflix subscription within the app on iOS. It just isn't there.
Fastmail is another app already on the App store that requires an account (but has no option to sign up within the app itself). Salesforce Inbox and GMail are two other email apps currently on the App store that do not allow the user to sign up within the app itself.
(An aside about Gmail: I'm now receiving almost-daily emails from Google
> Your Gmail is full. Get more storage now with a Google One membership. Plans start at $1.99 a month.
How much of that money do you think Apple would like to get? How much of that money goes to Apple? None. That is not implemented as an in-app purchase. )