According to the Sintech website as per this page http://www.pc-adapter.net/products/747.html it only supports the 2013 model cards. However this maybe more a matter of omission due to the adapter having been released before the 2015 model MacBook Pro.
I would personally feel that if the number of pins remains the same i.e. 12+16 according to the Sintech webpage not 12+18 as you said then it should still work. My reason being that these adapter cards have pretty much no active circuitry on them that would therefore possibly need changing, they are literally just a connector converter and pass the PCI signalling straight through.
It is however worth noting that the new MacBook 12" model - the one with just the single USB 3.1 type C connector, uses a totally different type of SSD. Previous MacBooks including the 2013 model discussed above use an NGFF type SSD, NGFF stands for Next Generation Form Factor. The new MacBook 12" uses an NVMe type of SSD. Both are PCIe based SSDs but NVMe ones use a new protocol which is specifically designed for SSDs and therefore supposedly far more efficient. NGFF use the old AHCI protocol which was originally designed for USB connected hard disks. So there is much more of a possibility that an NVMe SSD even if it fitted might not work, and without having seen a picture of one I don't know if it has a different connector.
Update: the MacBook 12" SSD is an NVMe one but it is actually built-in to the logic board rather than being a blade. I would expect future Macs including future Mac Pros to switch to NVMe SSDs and at that point we might see a blade version. Based on this any Apple SSD blade is currently going to be an NGFF style and therefore highly likely to still work with the above Sintech adapter. Here is another different webpage confirming 2014 model SSDs still work. http://eshop.sintech.cn/20132014-macbook-pro-air-ssd-to-pcie-4x-adapter-p-1026.h tml
As you can see from one of the pictures the actual PCIe adapter has pretty much nothing on it other than a socket.
