Except the policy doesn't create the trend. The Tesla app won't work if you don't own a Tesla. The Wells Fargo app won't work if you don't have a Wells Fargo account. These apps have external requirements by their very nature, the App Store policy does not change that.

Having a wells fargo bank account is not the same as having a wells fargo online user account.

You get the first, then sign up on the website or android app.

Try again.

You can't sign up for a Wells Fargo online user account without first having a Wells Fargo bank account. Therefore there is an external requirement.

Update: I just double checked. The Wells Fargo Android app and Wells Fargo iOS apps behave identically. You are immediately presented with a login screen upon launch. There is no signup flow inside either app but you can click a link that opens a web browser with a link to the registration page.

Your claim that the App Store policy is responsible for this trend does not hold up, considering the Android version is not subject to the same policy but behaves the exact same way anyway.

Update 2: Checked a few more apps. The Android versions of Github and Fastmail don't allow you to sign up in the app either. You are only presented with a login screen just like the iOS versions.

They're likely all webview apps with a single codebase though. If they remove it for Apple, it would likely be gone for Android as well.

Don't remember if they had a link for signups before though, just pointing out that you can't deduce that point from given facts.

(Nor is there proof for the original thesis, it's all speculation at this point)

Fair enough, but it's not like they couldn't provide a more direct signup option in the Android version if they wanted to. The fact that they didn't is telling, IMO.

To circle back to the original point, blaming Apple for the "trend" of not being able to sign up inside of apps is clearly nonsensical.