I always knew this day would come. I purchase this app when I got my first iPad in 2012, and since then, have been using it for work (as a teacher) every single day. $9-10, over a decade, that's barely a dollar a year. Crazy value, and it was also clear that this model would not be sustainable for the developer ultimately.
As with any paid app, the question at the back of your head ought to be - what happens if it every goes subscription-based some day. Do I stick around, or if not, what's my plan B with regards to not only alternatives but also any data residing in the app.
That said, whether this move contravenes any App Store rule or not, I literally couldn't care less. I am sticking around, I am happy to continue paying if it means the developer continues to support Notability for a good many years to come and well, I guess that makes 18? subscriptions for me right now?