This happens all the time with eg Google own apps. You might want to check your Play Store from a browser to see whether you still have these apps there.
It might be that your phone is set to automatically update apps, and during update, these apps were deleted if their developer decided so. Contact app's developer for more info.
Those apps are still available. I just couldn't install them. They always fail.
I tried to install via Google Play or the apk files. None works.
How could I backup the data and settings of those apps?
I browsed the phone folders from the computer (Windows). Some folders are empty. I guess the files of those folders are hidden. I couldn't backup the data.