The Android Market lets you depreciate old versions of Android now for your apps by leaving up the last compatible apk for them while newer devices get the latest version of your app.
With that, you can either let the old version die slowly as users upgrade devices. I haven't attempted it, but I assume you can still somehow push bugfixes for the older version without newer devices also getting it.
Backporting bug fixes or new features? I wouldn't be going out of my way to try adding new features to 5 year old versions of Android. If a user reports a crash for 2.3, sure, I will fix it. I know I have a fair amount of devices showing as Android 2.3 for one of my apps, but I've noticed most of them seem to be inactive. The app in question is a tool for testers and power users, so I would expect many of them to keep old devices around for fun/hobby, but not really using them actively.
Just something you might want to investigate. You may not have as many actively using 2.3 as you might think. Creating a new branch for active build really isn't that much complexity and then a branch for legacy when the legacy is just maintenance mode only.
We are talking only about people who installed the latest version of the app – so they had to be online, and had updates activated, within of the past few weeks.
About 25% of our users are below ICS even. Yes, one quarter of our users are on Gingerbread.
And then the rest is shared equally between JB, KK and L.