I examined both RC versions of bash and readline, and it turns out that the KEYWORDS are disabled in those. If that is -always- the case, we could just have IUSE='+system-readline' in any version regardless if it's a release or not, since users would not be able to install it anyway, unless they apply 'app-shells/bash **' to /etc/portage/package.keywords. If they do, then that means they're prepared for the risk, and should know when to use the use flag.
I created another example for this. This time, the feature can still be disabled, but only if there is no counter-package of readline for the targetted version of bash, which may happen in alpha or beta versions; not to mention 9999 (and 99999), which I believe should be easy to implement now after this. -- konsolebox
bash-4.4-r1.ebuild
Description: Binary data
