Hi Dennis,
Firstly, I'd like to note that I'm not an Apple employee, nor am I employed by any company affiliated with Apple. It is not my job and I am not paid to assist anyone on this forum. It is a place where users help other users. On occasion, Apple employees may assist users, but they will have a notice below their avatar.
If you would like to chat to an apple support representative, you may do so at support.apple.com
It's not really feasible for Apple to advice customers of deprecations. Emailing every user with a list of deprecated libraries and methods is a silly idea. Nobody would understand what most (if any) of it means. So, they don't. Even if you did understand it, you wouldn't know which apps use these and may have issues because they're removed. It may also foster an idea that updates are bad, when they're not. They bring great new features and improvements.
Your experience at the Apple Store is understandably frustrating. This really isn't the kind of experience employees and the company want you to have. Being a former Apple Store employee, I can tell you that the majority of employees don't know RAM from ROM. But why should they? They're retail employees. Nobody would walk in to GAP and ask how jeans are stitched.
Additionally, apps aren't a part of the retail store experience at all, their sold through a separate online marketplace. iTunes support, or the developer's own support should be sought in these circumstances.
Either way, the problem is with the developer. I can't tell what exactly has been removed to cause the app to become unresponsive in this way, or to say how long ago Apple marked it for removal, but it's safe to say it was at least June, when Apple held their WWDC keynote and released specifications on iOS9.
The developer has had at least 3 months, but, up to just over 2 years, to update their app in preparation.
For most developers, the update will be simple and painless, simply searching through their code for anything deprecated and replacing it, or running the app in debug mode and seeing where it crashes.
Again, I'll stress that you should email the developer, as this is entirely down to them to fix. It should be easy, and they really should have done it by now. It's somewhat absurd that they haven't.
Best Regards,
Connor