Just because the iPod Touch 5 and iPad mini 1 are still being sold doesn't mean they have to support iOS 9. The iPod touch 4 was discontinued in May of 2013, just one month before iOS 7 was announced. The iPod Touch 4 did not support iOS 7. I think the same thing will happen with the iPod Touch 5 and iPad mini 1. They will be pulled just in time for the WWDC event in June where iOS 9 will be unveiled. iOS 9, assuming it is not mostly based around performance and bug fixes, will support the following devices, I think:
iPhone 5, iPhone 5C, iPhone 5S, iPhone 6, iPhone 6+, iPhone 6S, iPhone 6S+, (iPhone 6C if that happens)
iPad 4th generation, iPad Air, iPad Air 2, iPad Air 3, iPad mini 2, iPad mini 3, iPad mini 4.
I do not believe a new iPod Touch will happen, sadly. I hate to see it go, but I don't think it is very enticing anymore for Apple, sales for the iPod Touch have been rapidly declining since 2009.
Now, what do you guys think about the iPad 3? As you may know, it has an A5X chip. Twice the GPU cores as the A5 found in the iPad 2, and also twice the RAM at 1GB which matches the iPhone 5 and newer, and iPad 4 - Air. In the past, iOS devices have lost support due to RAM constraints. (i.e, the iPod 4 stayed with iOS 6 due to only having 256mb of RAM while the iPhone 4 had 512mb) Anyway, I personally think the iPad 3 will stay with iOS 8. Yes it has twice the RAM and GPU cores but after the taxing retina display, I think it all levels out. All A5 devices of any kind will be dropped in iOS 9 in my opinion.
If iOS 9 is mainly based around performance and bug fixes, all devices running iOS 8 should run iOS 9, however I doubt iOS 9 will be a polishing break. Just a gut feeling, even though I would love that.
There will be no version of iOS where iPhone 4S is left out but iPod 5/iPad mini 1 aren't. They have the same chipsets, that just isn't happening, even though the iPod 5 and iPad mini are still being sold after the iPhone 4S has been discontinued. Just no. This should be obvious.