Bruh, really? I can understand why you mistakenly put Sion low on the jungle tier list as his win rate in the jungle is super low this patch for reasons I can't figure out, but why put Sion in the lowest rank of support? He has the 2nd highest winrate support. Sion, in my opinion, should always be put just under god tier. His amazing CC makes him good support, his tankyness makes him a good top laner, and both combined make him a good jungler. Clearly you have only ever played with absolute trash first time Sions. If a Sion has level 5 or above mastery they are going to smash everyone into the ground unless another lane snowballs extremely hard.
Now allow me to breakdown every amazing element of my boy Sion:
Passive Sion's passive is overall great. In lane, Its main use is to get revenge on the enemy laner if they kill you to stop their snowballing, meaning you can win lane with a few deaths. In the jungle it removes the possibility of the embarrassing "I couldn't get red buff because I was executed." Late game this passive defines Sion's playstyle: Go in and don't leave until they all die. Even after Sion dies after 20 years, he keeps fighting. Buying a Guardian Angel makes this all the more hilarious and effective at the same time.
Q Sion's Q is great due to the massive CC it brings. Sion is always in the fray, meaning that landing it with a knock up is pretty easy in teamfights. It also deals a ton of damage if you build something with a lot of attack damage.
W Active This is just a shield that protects you for up to above 10% of your health and then deals damage if the shield is not popped. Nothing special, but still good.
W Passive I love this passive. If you are farming correctly, lategame Sion will have over 700 free health just from this passive.
E A good slow for getting enemies under your Q and W active. Especially good for teamfights that happen near minions.
R This is quite possibly one of the best ultimates int he game if you can land it. As an engage, its great because it lands you right into the center of the fight from across the map, dealing a ton of damage and CC. If you just happen to have a Yasuo on your team, he will love you. Sion's ult can also be used as a great escape, with him being immune to CC. It can even chase down kills if someone almost escapes from a fight with just a bit of health.
TLDR, Sion is good pretty much everywhere and I can't come up with any idea as to why in 2 roles you put him as bottom of the barrel and in his main role as being just above the bottom