Why Sion's Decimating Smash is Hidden while he is in bush?

It's the same reason why Teemo's mushroom doesn't make an explosion when you enter a bush.

Why does 16 stack Cho get to hide in a bush? It's a game mechanic, not really a game aesthetic

A better question is why it can still hit you so far out from the indicator.

https://nexus.leagueoflegends.com/en-us/2017/11/ask-riot-victorious-champions/

Why are some spells (like sion knockup, nunu ult, zilean bomb, etc.) invisible to the enemy when cast in/from a bush while other similar aoe spells are not? A The general rule for brush visibility is: “When I damage an enemy, I am revealed.” We make sure to follow this rule always because we think understanding and responding to direct damage or spell hits is an important part of LoL’s clear gameplay. In cases where the spell is casting but no damage has happened, we take a less uniform approach.

Why we would not show an indicator:

We think the spell is risky or limited enough that allowing for the element of surprise adds a reasonable use case. For example, Nunu ult has a significant cooldown and a very long, easily interrupted channel. Allowing Nunu to use brush to his advantage is a strong pairing that we think brings out good use cases rather than unfair ones.

Why we would show an indicator:

When a spell is very low cost, low risk, or high range. Some spells would project far too much threat too frequently from brush if they could be cast without a reveal. Xerath Q is a good example here—if he could repeatedly cast invisible Qs from brush, we think it would degrade the overall play experience rather than enhance it.

RIOTSCRUFFY Lead Champion Designer