This is an important issue as its a very obvious question that anyone wanting to install plone 5 will ask (and my clients have asked already). It's also not easy for us to answer on a ongoing basis without CI testing of versions. I'm not sure who is the best person to ask about this.
As for screen readers, versions of JAWS from 2008 support more modern versions of IE; and since JAWS gained ARIA support in 2010 people have upgraded a long time ago, so the dreaded IE8 can probably go.
I'd still test on IE9 as there are still organisations out there using it, but since official support for it is dead (and that fact was well announced upon them by MS and their vendors) these organisations are playing a dangerous game.