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If its not too much trouble i think it would be very fun to see an FFA where we could use all of the official pokemon we want to use that aren't limited to GEN8
I asked about this on Showdown's subreddit, and the reason I was given is that coupling the already limited-use Free-for-All format with the divisive National Dex formatting may drive people away from the metagame entirely.
It does seem rather odd that a metagame based on an official battle format that was cut from Gen 8 uses Gen 8 rules. It definitely seems like it was intended for use with a National Dex in mind.