Well not every month but try and find a current app that will work on a 3G or previous, maybe 1 in 40 or worse.
This is not an Apple issue really. It's up to the app developers to maintain app versions for older generation iPhones/iPods/iPads. But if the app developer does, there's actually a way to keep that app working just fine, even on an older generation device.
In case anyone's interested, here is how to download apps to an older device (assuming again, the app developer maintains that app in lower iOS versions):
Requirements for Downloading an Older Version of an App
To be able to download an older "legacy" version of an app, a few conditions must be met (more detailed explanations below):
- You must have purchased/downloaded the app before (see this workaround for iOS devices if you didn't)
- The app must have supported your older iOS or OS X version at one time
- You must be running iOS 4.2 or later, or OS X Mountain Lion (10.8) or later
NOTE: Not all apps offer older versions. It is up to the app developer.
If you are attempting to download the app for the first time, the App Store will
not offer the download of an older version. You must already own the app for the older version to be offered (see this
workaround for iOS devices if you don't).
If you do not already own the app, one workaround is to purchase/download the app from the iTunes Store on a Mac or PC using the same Apple ID you are using on your iOS device. The app will then be associated with your account and you should then be offered an older version if you attempt to download the app again on the iOS device. Do not try to install the copy downloaded in iTunes, as it is the newer, incompatible version. That step is only necessary to attain ownership of the app. You will need to access the App Store on your iOS device to be offered a compatible version.