One day I was being stupid. On that day, I discovered you can still purchase Halloween cases and keys. This can be done by attempting to open a case with only a case or a key.
Key Method: The game will prompt you to purchase a Halloween Case and redirects to the page where you can purchase said case.
Case Method: The game will prompt you to purchase a Halloween Case Key, which can be done on the spot.
I do not currently have any images, but you can try yourself. Either that or it’s been patched since the last time I was online…