I went to the first Halloween party in 2019 and 2018. It was in August both of those years. I left within a day or so of those parties. Some of the seasonal treats were only offered during the party. Others were offered during regular Magic Kingdom hours. From what I remember seeing, it seems like the treats which were sold at the Main Street Bakery (Starbucks) were the ones which were most likely to be sold during regular hours.
We haven't hit the date for the first party yet, so I would think it would be too early for people to report back.