You can still trade/talk to them, just not the usual way you were used to. Click on their planet and you'll see the Speak To button. Or, go into the diplomacy screen and talk to someone else, then click on the right arrow to the right of their name to scroll through all the people you can talk to and the minors still show up there.
That looks more like an oversight than intentional to me ... 
Why not make them more reluctant/greedy/careful/sneaky when trading instead of shutting them off completely?
On the other hand, why have minors as such? Just make them majors, but give them less bonuses and starting credits. Could be a new "minor" race trait: gives them less money and sets their bonuses internally back one AI level (so e. g. when they are set to "gifted" they get the bonuses from "normal"). That should be enough so that they develop much slower and appear as "minors" to the "major" races.
