How does Smogon/PS intend to handle Gen 8 National OU? Won't there be interactions that we won't know?
Case in point: Suppose a Pokémon with Slow Start is in play. Two turns go by. Three more to go before Slow Start wears off. The opponent sends in a Pokémon with Neutralizing Gas. Slow Start is now disabled. The opponent then switches out so Neutralizing Gas is no longer in effect. Does the turn count for Slow Start resume where it left off, or does it get reset back to five turns?
In past gens, we could either look at the game's code and see what the interaction would be, or we could test it ourselves by playing the games. But (spoilers) as there is no Pokémon with Slow Start programmed into Sword/Shield, this won't be possible here. And this is just one of the MANY interactions that won't be in Gen 8 (at least, at the start -- perhaps future Gen 8 games will have more Pokémon in them).
The only thing I can think of is to just let someone arbitrarily decide what the interaction should be. But in that case, who would do such a thing and how would they make their decision?
Now that we know all the abilities are in the game, it is possible to test mechanical interactions instead of making them up. As such, NatDex shall be an OM.
How, if at all, does this take Megas/Z-Moves and axed moves like Hidden Power and Pursuit into account?
Those are deleted mechanics. NatDex only assumes that all Pokemon can be transferred from USUM to SWSH. I'm sure there'll be a Pet Mod of some sort with all deleted mechanics.