So you couldn't even get to the point where you can send an email without first falling in line with IAP.
And according to my understanding of the Spotify timeline, and I haven't see this because I haven't published to the platform in a while, Apple now prohibits you from targeting your iOS users with upgrade communications (even if they didn't sign up via iOS, which they didn't since that's prohibited).
(An "email me a payment link" button would almost certainly get rejected by Apple)
It could even be 2 digits (the same 2 digits for everyone), you’re just looking to get their eyes on the email letting them know additional paid subscriptions are available on your website.
I think you'd end up confusing users. Most people will go to their email expecting to see a verification code in big bold type. If you make the code less obvious, they'll get frustrated ("I came here to get a code, why am I now doing something else?") It's awful UX at a particularly business-critical moment.
And this is all assuming Apple doesn't see through your ploy and just reject the app.
This is especially damaging to small companies. Most people have heard of Spotify, Netflix, Pandora, etc. So it's somewhat natural for them (if not a bit akward) to go to the website if they can't get premium in the app.
But for small companies that connection isn't immediately obvious. In my company for instance, we aren't allowed to put ANY reference to our website in the app (as it relates to purchasing premium). As a result when a user signs up, my only option is IAP. I cannot just assume they will naturally go to the website.