Hi,
I think there's two separate things to consider here:
- When will new apps stop appearing that are compatible with iOS 9 ?
- When will new apps stop appearing that are compatible with the iPod touch 5 specifically ?
These are definitely related, but not quite the same thing. There are other devices stuck on iOS 9 (like the iPad 3 for example) which have 1GB of RAM instead of the 512MB of the iPod touch 5. In theory, newly-released apps supporting iOS 9 that need more than half a gigabyte of RAM could continue to run on 1GB devices like iPad 3, whilst then leaving 512MB devices like the iPod touch 5 and iPhone 4S behind.
The bigger question is: how long will new apps be able to support iOS 9 at all ? There are signs in iOS 10 that in the future 32-bit apps will no longer be supported in iOS. This would mean newly-released apps would be unable to run on everything from the original iPhone and iPad right through to iPad 4 and iPhone 5 (systems based on the A6 chip and earlier, basically). If that happens (or more likely, when that happens) your iPod touch 5 would definitely have reached the end of the line, with no new approved apps on the app store being permitted to support it.
We don't of course yet know if 32-bit app support is being totally dropped as soon as iOS 11, but on 64-bit iOS 10.x devices if you open a 32-bit app you get a pop-up dialog box that tells you that unless this app is updated it will not work in a future version of iOS. The wording of that warning has gotten stronger as the lifecycle of iOS 10 has continued, which leads many people (myself amongst them) to suspect that the 32-bit axe will indeed fall in iOS 11.
But then, there's one last point to consider here: if you don't really install a lot of new apps, and you have a working set of apps on your iPod touch 5 that work for you and work well enough that you don't need to upgrade, then you don't really have to worry. If your device works well for you, it will continue to do so. But if you're finding it a struggle, and new apps are not performing acceptably for you, this is definitely only going to get worse as time goes on.
Hope this helps anyway !