Q: app not available anymore, can I restore data from encrypted backup?

It seems I do not. It appears in the list of apps in iTunes, but when I try to install it, it is not available / cannot be found. So I thought it was in the backup, but it is not. And I cannot just look for files called ChildGrowth.ip* to see if somehow it ended up elsewhere on the computer because I encrypted the backup, so none of the files have meaningful names. Thanks for the suggestion. 

Digging further, I found the list of Mobile Applications under iTunes/iTunes Media (not in the encrypted backup). But the app in question is not there, and in fact there is no app in the list with a time stamp more recent than December 2014. So something has been wrong with my backups for a long time (probably I got careless about hitting Yes to the question about transferring purchases to this computer because I assumed I would always be able to re-download them from the App Store...).

... and finally, happy ending, I found a whole other directory of .ipa files from 2012 in a Mobile Applications folder that was directly under iTunes, and not in iTunes Media. When I copied ChildGrowth.ipa from there to the more recent Mobile Applications directory, it restored fine, and all the data are intact. So thank you for pushing me in the right direction. All is well, and some clean-up is in order for the backup computer.