No information on MGM website as to them being discontinued. Meeting at Midnight title was as a individual disc. It may be a Movies Unlimited choice(by TCM who owns them now). As to the rest of the series being released one never knows.....
I do not know about the discontinuation of Chanthology, however if you are looking to purchase, I recently bought mine at Barnes and Noble. Not cheap by any measure, but a "necessary" purchase nontheless!
If this proves to be so, I suppose that we should remember that this DVD box set has been available for over five years, now. That's a pretty good run. For years to come, we will probably be able to obtain them in both used or in unopened condition.