Soldier Eligibility for "Improve Combat Intelligence" in X-COM 2 WotC
Solution 1:
I think I figured it out. All the Faction Soldiers are apparently ineligible — as are Psi Operatives and Sparks. With the latter two it kinda makes sense since both work a bit different the usual soldiers anyways.
So eligible are just the base class soldiers which have not yet reached Savant level combat intelligence and who are not tired or wounded — and of course fulfill the prerequisites of the Covert Operation itself.