As of 10.10.3 Apple does apparently support MST* on certain hardware:
These Mac computers support multi-stream transport (MST) displays at 60 Hz:
- MacBook Pro (Retina, 15-inch, Late 2013) and later,
- MacBook Pro (Retina, 13-inch, Early 2015) and later,
- Mac Pro (Late 2013),
- iMac (Retina 5K, 27-inch, Late 2014) and later
*Unfortunately it does appear to be limited to an MST display that relies on multiple streams like the 4K MST Displays mentioned further down on the Apple Support page, and not MST for chaining multiple DP1.2 Displays as I haven't been able to find any information about this being possible.
From my own testing with the new MacBook Pro 2016 13" with two thunderbolts ports, a StarTech DP 1.2 USB Type-C to DisplayPort cable, two Dell U2414H and a standard DisplayPort to DisplayPort cable to chain with, the display on the end of the chain just mirrors the other display. This is with DisplayPort 1.2 enabled in the settings on both displays.
Which seems to confirm that MST DisplayPort daisy chaining on OSX 10.12.2 on the 2016 MacBook Pro line is unsupported, with no sign of it getting supported anytime soon.