Shen needs a base armor increase like Jarvan
Since he got his healing nerfed it has left him weaker (especially against matchups where he was considered good in) because the health regain is such a significant part of his trading power and lane dominace (if he does it right).
I believe uping his armor to 28 from 25.7 (26 in game) is a nice way to give him that "inherent sustain" without making him an "immovable wall against certain matchups.
It would just be a partial reversion to the nerf he got yrs ago anys (from 23 down to 19) which would be 30 armor as of our current growth stat rework.