I've been saying Sion can support since day 1 of his rework. I think I even made a thread about it. In bot lane you want to push, which he can do very well with his E. He has peel with his Q, he can be effective without CS, he brings a lot of CC, and if he dies in the middle of a team fight he can still potentially put a lot of damage down since his passive does 10% max health damage on hit.
Top tier support? Probably not, but he may be better than Taric at this point. He's definitely at least as good as Gragas and other similar off meta support picks.