I checked the Japanese version of the site, the maintenance begins at midnight over there on the 12th (which will be the 11th for us in America).
To confirm, the wording they used in Japanese definitely means they are ceasing to sell the following products (以下の商品の販売を終了させていただきます).
VOCALOID4 Editor upgrade version
VOCALOID4 Editor for Cubase upgrade version
VY1V4
VY1V4 upgrade version
CYBER DIVA
CYBER SONGMAN
VY2V3
Rana V4 upgrade version
Fukase starter pack
Tone Rion V4 upgrade version
v4 flower upgrade version
VOCALOID SHOP Selection (giant pack of "VY2V3", "ZOLA PROJECT", "AOKI LAPIS", "MERLI", "Mew", "AnonKanon", "galaco")
VOCALOID SHOP Selection Starter Pack for Cubase
VOCALOID VY2 + ZOLA Starer Pack
VOCALOID VY2 + ZOLA Starer Pack for Cubase
VOCALOID Fiery Starter Pack (I don't see this in the store, I don't know what it is)
VOCALOID Fiery Starter Pack for Cubase
V4 VocaListener
V4 VocaListener upgrade version
It must be V5. It looks like they are mainly getting rid of upgrade versions and large packs of Vocaloids. But does that mean they are getting rid of the individual products in the packs? The store currently sells V3 and V4 products. If it's going to be V5, then it would make sense to only sell V4 and V5 products (axing the V3's).
The shop only sells the current version of VocaListener (you can't buy V3 VocaListener for example). They are either going to release V5 VocaListener or stop making it altogether (someone on the Discord said AIST who made the plugin isn't working with Yamaha anymore, but I haven't tried to confirm). If VY1 is popular, maybe they would make a V5 of her. But it seems strange that Cyber Songman and Diva are on the doomed list. Songman was on my wish list, looks like I'll have to fork over my money earlier than I had anticipated... OTL
Edit: Ug, why would they discontinue the Cybers if their YouTube tutorial series is based on them?!?!