The only way it's ever going to get used.... is if it does damage, heals, gives armor/mr, gives cdr, or gives some other stat.
No one is going to take a keystone for a slow. EVER.
It's also completely useless 90% of the time past lane because odds are someone else has a slow already.. and slows don't stack.
MAYBE if the slow stacked... people would use it.
Here is a list of champions that already have a slow.... and Glacial aug doesn't stack with any of these. Just try to build a 5 man team without using any of these champs. That also doesn't have a Mallet, Rylai's, Exhaust, Gunblade, GLP, or BOTRK.
Note list might be slightly outdated, but I ain't about to make on myself.
Aatrox
Akali
Anivia
Ashe
Aurelion Sol
Azir
Bard
Brand
Braum
Caitlyn
Camille
Cassiopeia
Cho'Gath
Corki
Darius
Diana
Dr. Mundo
Draven
Ekko
Evelynn
Fiora
Fizz
Gangplank
Gnar
Gragas
Graves
Heimerdinger
Illaoi
Irelia
Ivern
Janna
Jarvan IV
Jayce
Jhin
Jinx
Kalista
Karma
Karthus
Kassadin
Kayle
Kayn
Kha'Zix
Kindred
Kled
Kog'Maw
Lee Sin
Leona
Lissandra
Lulu
Lux
Malphite
Maokai
Miss Fortune
Morgana
Nami
Nasus
Nautilus
Nunu
Olaf
Orianna
Ornn
Pantheon
Poppy
Pyke
Quinn
Rammus
Rengar
Rumble
Sejuani
Shaco
S cont.
Shen
Singed
Sion
Skarner
Sona
Soraka
Swain
Syndra
Tahm Kench
Taliyah
Talon
Teemo
Thresh
Tristana
Trundle
Tryndamere
Twisted Fate
Twitch
Urgot
Varus
Vel'Koz
Viktor
Vladimir
Volibear
Xerath
Xin Zhao
Yorick
Zac
Zed
Ziggs
Zilean
Zoe
Zyra