It's a Google thing, not necessarily an app thing. Apps will have a minimum SDK level (basically the oldest Android version the app can run on) and a target SDK level (the Android version that the app is optimised for, which is usually the latest Android version). Google keeps increasing the SDK level requirements for apps to be published on the Play Store, so if an app is too old and hasn't received any updates, you will see that message. But chances are that the app will run just fine if you install it from an APK file.
It may seem ridiculous but it's good because it prevents people from installing super old apps, which may leave room for exploits in modern versions of Android.
Right of course. But I doubt they developed and published their app in-house is my point. If they contracted some developer, who knows if the ACLU themselves or some formerly contracted developer gets notified that the app is "old" (if anyone at all is notified).
Edit: I was right. This is the app developer for the ACLU app you screenshotted - https://www.quadrant2.us/
I still don't understand why Nintendo don't make an official Game Boy emulator and then sell all the old games. I was a little bit too young for Zelda, and by the time I was old enough to really play it we've moved on to other platforms so I feel like there's a gap in my cultural knowledge that needs to be filled.
this is mostly a security thing, because old apps lack granular permission system that modern apps have.
(you can still pick perms on first start but it's kinda messy and breaks apps
I have seen it too many times. I tried to install many apps from the google play store and it shown me that it was unavailable. When i installed it from an APK file, it was on the play store, and it would work fine.