Experimental App Store app downgrader / installer with TrollStore.
For downloading older versions you can choose between entering a app's older version's identifier manually, or getting them from a API. You might consider getting them from the API a invasion of privacy, as it needs to also send the app's app id you want to download with the request. If you want to use this app without using a external API except the App Store, you can just enter a id manually. You can get them from ipatool-py, and numerous other places.