idk how serebii.net does it, but they're always on top of the most recent, factual news, so yes, i think we would've heard something. I'm not even the one who started saying that new types are unnecessary; people in this thread have made good points that our current 17-type system balances out well. and think about this: new types were last added in Gen II and it's been argued that it was to fix the uber psychic-type and other reasons. if they wanted to.add a new type, then they would've done back then, immediately after Gen I.
and yes, it's important to note that trustworthy sources HAVE NOT mentioned new types. when people speculate that Sylveon is normal or flying or dragon or whatever, it's okay because we can all agree that Sylveon HAS to be SOME type. but when you start to make up Pokemon types, then it gets a bit ridiculous. there is no evidence for your made-up types. people did it with reshiram, too. light types has been nothing but a rumor for years. it's fake. i can logically speculate that Sylveon will be a normal-type because there is evidence that Sylveon is a real Pokemon (as confirmed by Serebii.net) and there is evidence that normal-type is a real type. but there is NO evidence that light-type is nothing more than a rumor. i do think there could possibly be new types someday, but I'm not gonna.say illogical stuff like “it will probably be a light type". it's just illogical