To each their own, but I find this decision really misguided.
It's her money, not mine, so whatever, but l do not expect her to turn a profit in, rather the opposite.
In my view, the cross section of "IfR" users and people willing to subscribe monthly is rather small (especially if the money mostly goes to reddit - assuming I could afford it, I, for instance, would rather fund an open system like Lemmy).
And if Apollo's dev Christian Selig decided that it wasn't worth it with an already established paying user base, who already has a strong culture of subscriptions and exaggerated pricings, and one of the highest volume of users, at what probably was the peak usage of the platform; I don't see how a small app like IfR can survive.
That, or Christian made a pretty expensive mistake...
It's OAuth. Each user that allows access to the app will have an individual token only valid for that app and only for that user's account. Either the developer or the user can revoke that token at any time.
All the dev has to do is to not create/send a token to the user until they subscribe, then revoke that user's token if the subscription expires.