I think Sivir
would be a viable support.Here's why,she has a spammable q once she gets a chalice 
or some mana item, her q is one of the most damaging spells in the game and her ult is really easy to use if a jungle comes or to disengage or engage from a teamfight, she also has pretty good aoe with q and w and she can save her carry pre 6 if your good enough to body block a spell with your shield.
Sivir will never be a viable support. No cc, no utility except for her ult, no way to peel for an adc. Why do you want Sivir to play support, just for her ult? The active of Talisman of Ascension does the same thing and it's one of the three gp10 items for supports.
You also do no damage with no items and have no CC.
If you just want to spam harass, you might as well use Lulu or Sona.
Sivir is so so so so useless when she has no CC compared to a Leona or Braum. Even more useless when she can't even do shit to save her ADC from a gank.
Anyone can have a Sivir ult if they build Talisman of Ascension. There's nothing Sivir can bring to the table that a meta support can't do better. Her Q will be useless mid and late game without damage items. Mikael's Crucible is better than her E. Anything Sivir does can be accomplished just with items alone.
Most AD supports I have Seen, wined up being kill lanes. And these kill lanes are usually cheese that won't work on a higher Elo.
Part of the reason I say this is that you end up with a Double AD bot lane, which becomes easy to itemize against. Against said lane I can go with a Tank Support and stack Early Armor. Even with a poke support, I can take a Frozen heart instead of my core Mana Item and still do fair.
The base damage on her Q won't stick up throughout the game, and her W will end up doing practically no damage. The spellshield only has self-protection, so it's also going to provide you with very little value as the game goes on. All you'll really be contributing is the ultimate, and you can itemize for that kind of speed boost on a support.
You could make it work, but only so far as you can technically force anything to work. She would always be worse than another option you could take. As far as I'm concerned, that means she wouldn't be viable. Do it if you think it will be fun, but don't expect good results.
i'd rather have support ashe or caitlyn. sivir has no utility and needs damage in order to be useful.
