this has been troubling for some time now. Since I started to main Sion I always use to think that Sion lost his foot in the battle against Jarvan the first. BUT, in Sion's barbarian skin where its a skin a Sion BEFORE he even fought that war. So what happens to Sion's left foot? My friend told me it was just weird looking shin pad/Plate boot. BUT he still limps on his left foot in that skin. SO CONCLUSION. WHAT HAPPENED TO SIONS FOOT?!
Barbarian Sion isn't canon. By default no skins are.
Sion's left foot was lost in a meat grinder accident when he used to be on cooking duty.
Joke's on them, he never cleaned it out.
He stepped on a lego.
Sion would go get his foot... But he forgot where he put it. Its probably by his bed... But since rest is for living well...
Barbarian Sion doesnt look like the Canon Human Sion at all. Most of the skins arent canon.
http://apollo-na-uploads.s3.amazonaws.com/1411981321443/Head.jpg
he stepped on one of 
traps and chewed his foot off before she could headshot him for his entire hp bar
http://vignette3.wikia.nocookie.net/leagueoflegends/images/e/eb/Sion_Render.png/revision/latest?cb=20140925232855 http://news.cdn.leagueoflegends.com/public/images/articles/2014/november_2014/SSG/Sion_Concept_Comp_02_thumb.jpg
Look closely at his left leg, it's a peg-leg. This means he simply no longer has either the foot or the entire lower-leg on his left, as that's a prosthetic he's using. Hence why he limps on his left.
As for why the left foot/leg is missing, nobody knows atm. He could have lost it in battle while he was alive, maybe he just lost it because his corpse wasn't "whole" due to decomposition and other possible reasons, or maybe it's just that the resurrection may just not be perfect (I mean look at him, it's obviously not perfect) and so one of his leg/foot was not brought back. Who knows.
Barbarian Sion probably isn't canon, though. It's nice to imagine how Sion may have looked like when he was alive and kicking and on the front-line with the skin, but it most likely isn't canon with the League universe sadly.
he lost in a random fight he can't recall when
The person who drew the skin probably forgot that it was b4 the war....Pun of the day " i used to hate maths but then i realized decimals have a point"
He made the mistake of taking too many Crit Damage runes.
Now he has to kick with a stub.
