6e rules questions are in a sticky place right now. There is an errata process underway, but obviously nothing is available to the public as of yet.
Now the question can also be asked of 5th edition, and that's a context more easily discussed. The fact remains that in the chapter defining game concepts, there is no allowance in 5e for opposed tests to incorporate a threshold. As to whether that means "it doesn't say there CAN be one" or "it doesn't say there CAN'T be one" is going to boil down to your philosophical understanding of how rules work. Personally, as far as I'm concerned, it means "it doesn't say there CAN be one" ergo no opposed test may ever incorporate a threshold. And IMO this is the basis, for example, of why Indirect Area Combat spells (which incorporate a success test with a threshold rather than an opposed test with a dodge pool) can never be dodged, despite the 5e rulebook making a couple oblique references to dodging fireballs.
So. For 6th edition: You'll have to wait and see if the notation for Opposed tests includes the possibility of a threshold. And then keep an eye out for the possibility of errata changing or clarifying whatever the printed word happens to say on this specific topic. In the meanwhile, for the QSR: You'll have to decide for yourself if it "doesn't say there CAN be a threshold" or it "doesn't say there CAN'T be a threshold".