Show me an example where a fight has been proved non-canonical, I know unknown was canon.
I believe as of now, there's no way of saying for certain if an optional fight is canon or non-canon (except for the secret fight against Xemnas in KHFM). It's hard to fit the fights into any of those two categories.
It will only be canon if it's touched upon in future games (for example, if in KH3 YX is confirmed to really have time-travelled and appeared at LoD after its destruction) or if Nomura himself states so. However, it will only be non-canon if it falls completely and obviously outside of what's considered canon, such as if the fight against VLS happened before the final showdown at Keyblade Graveyard (but this isn't the case).
I'm sorry if my understanding is erroneous.