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.
The iPod Touch 4 was different. For many reasons. It was the only device left with 256MB of ram. There are 4 devices with 512 MB of ram left supported, two are currently for sale. The iPod Touch 4 had a very very small share of iOS device usage, where as the iPad Mini 1 holds over 20 percent of the iPad usage share (45 percent if you count the iPad 2 which is the same device in a different package performance wise).
The other thing to ponder is that originally iOS 7 development started under Scott Forstall, and continued into early 2013, when it was taken over by Johnny Ive - Thats when it gained the new design and power sapping features, quite late in the game. When the decision to keep the iPod Touch 4 around was made, presumably iOS 7 was going to be far less resource heavy. By contrast, we know iOS 9 was in testing ages ago, so Apple probably made the decision to keep not 1 but 2 A5 devices around because it knows that iOS 9 will run on them. Just my guesses based on the rumours we heard during iOS 7 development.