As with all Fremium games, anything the devs release whether it's stuff bought with in-game money or premium items, they are yours to keep. This includes any or all "Limited Time" only items. The "Limited Time" banner simply implies that these items whether they are bought with in-game cash, donuts or treats are ours to keep just as long as we got them in time. After Halloween, these items will retire and any chance to have gotten them will be gone. Hence the Limited Time. I'm pretty sure there will be another set of "Limited Time" items when xmas rolls around and they release a xmas themed content update.
As manufract already posted a link confirming what we already knew.
The only thing I can imagine is that they may change certain things such as receiving treats from jobs or from buildings. I'm also hoping the Zombies stick around and start dropping money instead of treats
