[jacanuck]

I just purchased the mi box s in the hope to play (passthrough to receiver) DTS-HD and Dolby TrueHD ... is there any way to achieve this with custom firmware/ software downgrade or some other way?

AMLogic Android devices running Oreo v8.1 or greater no longer has IEC61937 Kodi HD audio passthrough support in Firmware.
No idea why it was taken out.

The Nvidia Shield or the 2016 Mi Box (model MDZ-16-AB) still has IEC61937 support.  

So, a possible 8.1 upgrade for 2016 Mi Box will eventually 'break' the passthrough? 

Yes - in fact the developers on their Facebook page basically said that they weren't aware it was working on the 2016 mi box, and that they intended to remove it (citing a license problem).

This was when they changed all of the product documentation for the mi box S to remove mention of DTSHD or TrueHD, and now have a generic "dolby digital" mention.

This company plans to break your 2016 boxes, and lied about their current ones - boycott.