Dang it! I think I may be hurting this discussion more than helping. I forgot about the new update for 2021. So it turns out what I said the first time was kind of right. I'll edit and delete my posts accordingly.
With the 2021 rules, and AWD STU car (Golf R) would be able to substitute the haldex for another differential. As I said the first time, I'm not sure that's possible. My interpretation is that you can't substitute the open diff in the trans case for an aftermarket diff. That's not substitution, it's addition. The ruling on the Golf R w/ aftermarket diff can be found at the end of the published 2019 Solo Nationals results.
It's kind of interesting that the old rule had a provision for an AWD car with all three differential positions open and the new rule doesn't. To go back to the protest, the competitor basically thought the MK6 Golf R fell into that situation. He contentended that the Golf R had no LSD and because of that he could add one. But they ruled the haldex is an LSD. I agree with the ruling, but also don't care what anyone on this forum does to their car. I just suggest being open about things. But still it's interesting because the rule that was broken in the protest is now changed. Personally, I think the ruling would be the same if a front diff was to be added to an MK7 Golf R in STU with the 2021 rules.
2021 STH AWD rules say you can't do anything to the diff. You can't substitute or tune.