Installing Old App Versions to Old Android Devices and Planning for Legacy Users
Whilst most adults will have a reasonably up-to-date smartphone, the same can't be said for younger siblings or children that are just getting used to the responsibility of carrying a mobile phone, typically being handed down old phones as their parents or siblings upgrade.
One problem with this is that older devices might not support the most up-to-date operating systems and apps needed for things such as bus passes. Unfortunately, the Play Store doesn't provide a mechanism for installing the most recent compatible version of the app to your device, so side loading from unofficial sources becomes the only way to make these apps work on old devices.
In the remainder of this post, I'll explain the issue that made me need to do this in the first place, and provide a tutorial on how you can safely side load apps on to an old Android phone without risking the addition of malware or disabling key security features.