I recently discovered that on my new phone (the old one died suddenly because of eMMC failure) some purchased apps are no longer on Google Play. They are not incompatible with device; they are not listed at all.
I have some old backups by Helium Backup, but I am 100% sure that newer versions were released.
Is it somehow possible to download these apps? Most of them I purchased, but I can't find them even in "Library". I contacted their respective developers, but got no answer at all...
Unfortunately, the answer is no, nothing can be done.
Apps that are paid for via Google Play can only be installed and have the purchase validated by the Play Store. Even if you get the app from a third party, it will not be able to authenticate that you paid for it via the Play Store. If an app is removed from the Play Store, it will continue to be authenticated on your device as long as it is installed, but if the application is removed for any reason, it will no longer be available from the Play Store, and the purchase information is lost.
Is it somehow possible to download these apps? Most of them I purchased, but I can't find them even in "Library". I contacted their respective developers, but got no answer at all...
The app is not really the issue, although a variety of different methods are used, the most common one is that app is the same for the paid or free version, but when you pay for it, an authentication token is issued for the app on your device. When you remove the app, the token is removed as well. If the app has been removed from the Play Store, a new token will not be issued.
In most cases, the developer can do nothing anyway, a sympathetic dev MIGHT give you an alternative to use the paid version, but I do not believe they have a way to validate that you purchased it from the Play Store anyway.
You can try to find the app on a site such as apkmirror.com or apkpure.com. Download the .apk file for the app, and install it using adb. This is called "sideloading," and you can find instructions for it here or here.
