So does Propellerheads Reason still have a "Sound" to it in 2019?

[Etienne1973]

Propellerhead never said they had non-linearities in the Reason summing engine (nor does any other DAW, AFAIK).

Does the Harrison Mixbus DAW have non-linearities like Brainworx TMT?

Not that I'm aware. Brainworx TMT is "patent pending", which means noone else came up with something alike - that's why they asked to register the technology. AFAIK, all channels in Mixbus behave the exact same way, like in any other DAW.

Your are right. I own a Harrison Mixbus 5.1.0 license. Made a little test:

2 audio tracks with same audio file and 1 audio track phase flipped. EQ and Comp bypassed and no effect plugins. Result: Null

Brainworx TMT technology seems a temporary one and only winner concerning different non-linearities on individual tracks.

[sin night]

[...]PAN LAW between DAW[...]

I recall doing more than one export from Reason and trying to null those exports without success. Of course, pan law is out of question when you export a few times the very same project from the very same daw.
I guess I'll try again soon...

[Etienne1973]

Does Pan Law also come into play if I do a DAW summing null test with no tracks panned?