Well in theory, since the whole WW catalog is transferable, im assuming every item that was in WW is in here, including the full Mush set. meaning it must be back.
Yes, the mushroom set is back, but it's not a festival this time. You get it from picking mushrooms off the ground (next to trees) in November. I haven't seen any mushrooms yet though.
CHerry tree like said kiwidragon you will not have acorn festival because this festival in acww replace mushrooms finding in gc, and like it said before it's back. Good news it's not just for 10 or 15days but for all month long, so if you don't find a lot or any mushroom in one day don't panic, it's logical.