This is a large screen shot, but it looks like the off road mode is the only hill descent mode.
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It's great for proper off-road hill descents. I use it quite a bit for sketchy and steep descents over loose dirt, gravel, mud, and even snow. It's great at managing speeds - easy to set your desired speed - I typically use it within 3-8mph depending on conditions.
On-road, definitely downshift. The downside of the 2L 4cyl is it's not enough compression/engine braking and as the engine RPMs increase it will upshift. You don't want it to get stuck in 1st while gravity accelerates your T6 past redline. It sounds like Subaru's hill descent control is not as fine tuned for off-road where you want to keep speeds in the 3-8mph range.
The upside is with a T8 there's additional "engine" braking with B gear (for ERAD braking) and it's totally fine to lightly ride the brakes to increase ERAD braking for greater battery regen. I just wish I could dial up B's regen - it would be great if the mode selector would double as a way to dial it up or down.