I think I've seen cases of apps being unpublished for new purchases (even free ones) but still available for downloads of existing users in both Google's and Apple's stores.
I had recently unpublished an app of mine expecting it would still be available for purchasers, didn't work. Had to resort to the outrageous price trick.
That's strange–I wonder why unpublishing didn't work for you. I have seen several apps be taken off the store by various companies but remain on my list of Purchased apps on the App Store. I can still download the apps but not see the entry on the App Store when I tap on the app's name for details.
I think I've seen cases of apps being unpublished for new purchases (even free ones) but still available for downloads of existing users in both Google's and Apple's stores.