Substack is implementing their writer lock-in right now.
Writers, leave while you can.

Substack's new in-app payments frustrate publishers: "One option is terrible for us, the other is terrible for our readers"
The newsletter platform's latest update means that Apple users will have to purchase subscriptions via the Apple Store at inflated prices, and publishers will see delayed payouts. But it also undermines one of the platform's core selling points: migration of mailing lists