How is the new wording different?? It reads exactly like the old wording, to me.

Because it allows monsters to take actions that aren’t defined by hidden monstrosities?

I misread it when I thought it disallowed any Battle Action. I now think the “and which” phrasology makes it sufficiently clear that it’s only those actions you couldn’t have taken without hidden Monstrosities.

Ah, so that’s what this is all about. At least in this respect, I think the rule is already perfectly clear.

It doesn’t say “without having” so it opens up interpretation onto without what is being referred to as without. Is it “without using” etc

Oh, ok, I missed that you added “having”. Still, I think I am ok with the “having” being implicit, here. Otherwise, you could really go down the rabbit hole with these conjunctions.

Actually, it doesn’t even matter here. You could equally read the sentence as “Any Battle Actions which the Monster has performed and which they couldn’t have performed without using Hidden Monstrosities” or as “Any Battle Actions which the Monster has performed and which they couldn’t have performed without having Hidden Monstrosities”; the end result is that the Battle Actions shall be as if they had never happened provided that a) the Monster has performed a Battle Action that required having and/or using a Hidden Monstrosity, and b) the Monster did not have and/or use a Hidden Monstrosity. For the purposes of Battle Actions not relying on Hidden Monstrosities, the difference is immaterial (and the intention of the rule’s drafter was that there is no difference at all).

And of course the “and which” mentioned by TyGuy above is crucial: the Battle Actions that shall be as if they had never happened are specifically those that would have required having (or using, if you prefer) Hidden Monstrosities.