I was about to post regarding this issue. It has been well over a month. We have 'yet' to receive any clarification was to 'why' he is disabled in the mode. Theories floating around is because he makes the gamemode toxic? I highly doubt it entirely. Shaco requires skill to function effectively in that map. If we're disabling champions because they're toxic for the mode.
Why is
Teemo
Nunu
Heimer
Caitlyn
Jhin
TALON
Kha'Zix
Rengar
Evelyn
Twitch
Malzahar
Janna
Zac (pre-rework mind you)
Urgot
Just to name a few, have not been disabled? Each of these champions presents a mechanic that makes dealing with them in the mode absolutely disgusting. Take Kha'Zix as the best example next to Evelyn. Kha's passive goes into overdrive on that map. Evelyn's ability to scutterbutt around full nude shrouded in shadow, makes her back capping quite annoying.
Rengo doesn't need bushes in that map
Talon's E makes him slippery as fuck
Jhin is like Shaco, traps, traps everywhere!
Same issue as Jhin with Caitlyn. She just plops traps and sits at 1 point in a bush and camps.
Twitch can sneak around quite efficiently
Malzahar can solo the ascended at a certain point
Nunu can literally spoon feed his team the ascended
Heimer just cops a squat at a point
Teemo just puts shrooms everywhere and/or camps a single point.
So the reasoning behind just Shaco being disabled is quite perplexing. When there are several other champs that are much better if not more annoying than him. Plus after his recent nerf Shaco's boxes are about all he has left in that mode now.