You can sup with a spoon if you are good
Few basic rules for a sup champ
1 Must be able to be efficient without farm
2 Must be able to help bot and teammates in one or more categories:
Defense (es: shield, heal, etc)
Attack (es: peel and poke)
Utility (es: cc, buffs, handicap to enemy...)
2.a possibly be a good combination with bot
2.b and a counter against opponent botlane
If the champ is able to fulfill both points, it's good to go.
Sylas can work as surely Neeko do
He actually work better as sup and jg than any other role if we take in consideration his ult, in bot he can steal it from 2 champ while in jg he can steal it from anyone, so more refresh on his low cd ult
-Counters, since his kit foresee close combat he will suffer against ranged cc and mobile champs
-CC, 2 weaks but one is a hard cc
-Sustain, 60 / 80 / 100 / 120 / 140 (+ 40% AP) to 105 / 140 / 175 / 210 / 245 (+ 70% AP)
-Ult 85/50/15 + 200% of target ult cd, so as mentioned above he can steal more often in bot lane
He can excel against tank sup or immobile bot
His build rely heavily on ap and as a sup he will suffer some how for the low income but mostly for the sup item (ward and boots) but a good player can overcome with a good laning and Meja (one of the best item for attack sup)
Ult, is very dependent by the enemies, against some champ it will be quite useless, but in the lucky situation it will be ludicrous (85/50/15 with 40% cd of ults that have way longer cd will give the feeling to be in urf)