Question: doesn't this support thread indicate that MST *is* supported?
https://support.apple.com/en-us/HT206587
Technically, yes, but in that context Multi-Stream Transport is being used to send a 5K@60 video signal to a single display. Such a signal requires more bandwidth than a single "stream" in the DisplayPort protocol can provide, so it must be multiplexed across two DP "streams". DisplayPort daisy-chaining also uses multiple streams, but in a different way, such that multiple target displays can be addressed, with one "stream" assigned to each. This is a feature of the DisplayPort protocol (version 1.2 or later) operating natively on the cables involved, as opposed to being encapsulated within the Thunderbolt protocol. The way that Apple expects you to daisy-chain displays is via Thunderbolt, not via native DisplayPort. Unfortunately, this requires a separate Thunderbolt controller chip for each target display to demux the DisplayPort data from its Thunderbolt encapsulation.
As has been discussed earlier in this thread, this is strictly a software problem; the Windows OS (W10, or even W7) running on Mac hardware supports native DisplayPort daisy-chaining just fine. For whatever reasons, Apple has chosen not to include the same functionality in macOS. (Perhaps Apple has calculated that the potential usefulness of native DisplayPort daisy-chaining functionality would be outweighed by the confusion and support burden that might be involved in helping end-users puzzle out the distinctions between native DisplayPort and Thunderbolt-encapsulated DisplayPort.)
Having said that, I can't help but wonder if the necessary code that enables DP daisy-chaining in Windows or Linux device drivers could be transplanted into macOS via a kernel extension (kext), and made available to end-users who want that functionality badly enough to go looking for it. Such a solution would be of particular interest to owners of all the 2012 Mac mini models, and also of those 2011 Mac minis that feature the Radeon-based discrete graphics support. These computers are entirely capable of driving dual
2560x1440 (1440p/QHD) or
2560x1600 (WQXGA) displays via their one built-in DisplayPort/Thunderbolt connector, but without access to DP MST daisy-chaining, the only way to do this is to use TB daisy-chaining... which has generally meant deploying a pair of (relatively-expensive, and now long-discontinued) Apple Thunderbolt Displays, one piggy-backed to the other. Support for native DP MST daisy-chaining would allow the use of almost any two displays, provided that at least one of them supports that feature —
or the use of a single DP MST "hub" that would connect directly to each display, to include even HDMI displays that know nothing of DisplayPort.