The developer owns the intellectual property that is their app. Apple cannot do anything to modify that.
Like it or not, independent developers or companies are just that, independent. They own their IP and their product and they are solely responsible for it. And if the go out of business or walk away from their app and their customers, there is nothing anyone or any other company can do about that.
Apple is merely the vendor of third party apps. They vet them fir some minimal requirements in terms of safety and privacy and such, but that’s it. But they are not at all responsible for any other companies product or support for the product.
Estill Hill is still selling products in the App Store. You need to contact them for any assistance with one of their products. According to listings in the App Store, they have a hotmail account for email contact. Note though that none of their apps have been updated in 2 years or 4 years, so it looks like they are simply out of business.