Hey dujke,
when they say ionic is not supported, it is indeed meant that they do not test or support lower versions. They might secretly test, but they do not promise results on those devices or browsers. I’m actually doing a project for IE9 and up while desktop browser are not supported at all.
You just have to think about some things (like, position: fixed has limited support and you should never use it for mobile development, at least, not yet… maybe in a few years), and have to fix some issues that have nog been covered by ionic. IE9 renders some things pretty differently from other browsers, for example.
You on the other hand, have the luck that you are targeting mobile apps, which have a better support for most things. You probably won’t encounter much trouble, but you must test on older devices to ensure! There will be chance you have to redesign your application’s css or structure a bit, so i recommend buying an old android phone (could be cheaper then 70 euro’s nowadays) with android 2.x on it installed, if those browsers and your most recent version render your app well, you’re like 99% sure it will render fine on (almost) any android device there is. Of course, screen sizes I did not cover in this statement.
Same goes for iOS, if you want to support iOS, test different versions for it.
In any case, not supporting does not mean that the framework doesn’t work. It means that they do not promise that it will work, and it might be that the framework or parts of the framework might not work, or might not work as intended. Most issues would probably be solvable for yourself in your custom code, while others might not be as easy.
As a small example, ionic is really not IE8 or lower compatible since it does not bind to events in the old IE’s way, therefore I decided to drop support for IE8. In your case though, you probably won’t have to make such decisions and might (but I personally think there won’t be much) run into small problems where you have to write fixes for.
Biggest problem is, you might not be able to use updates of ionic since they might break earlier versions of android (or other platforms) that doe work with the current version. Keep that in mind when making a decision. It probably won’t be a big problem if you can’t update on existing apps though.
I could test on android 2.4 for you, but I don’t have an android 3.x device.
Hope my answer helps clarify things 
(also, if someone disagrees with what i say or think i missed something, feel free to point out, i might learn a thing or two as well)
U need to do a lot of custom coding/testing for this single extra platform…