For those who haven't seen the segment from Pokémon Smash, it had the team (cosplaying as Eeveelutions) trying to figure out Sylveon's type. First, Shoko (playing Sylveon) pulled out some Nunchucks....but then got shoved inside a drum by a big burly guy, implying Sylveon is not a Fighting type, and in fact is probably weak to them. Next, she broke a block of ice, which was taken as Sylveon being super effective against Ice tyes. Akiyama and the others suggested that she may be a Steel type as a result of these two results, and decided to test the idea. The method they came up with? Seeing if she resisted Poison by having her put her head in a box and expose her to some really bad smelling items. That...didn't work out so well for Shoko. Not clear if that was meant to imply Poison was super effective, or that it was just effective, but at least it was clear that Sylveon lacked the Poisin immunity of Steel.
(Yes, crazy stuff like this is typical of Smash)
TL;DR
Smash implied Sylveon is strong against Ice types, and weak against Poison and Fighting types
It should be noted though....there is the possibility this was horsing around. The Nunchuck thing is something that Shoko is known for. She's done it at a number of concerts, and she even did it early last week in a cameo on Akibaranger. So...how much of the "tests" were real hints, and how much was misdirection? Not 100% certain there.