A still from the official video (terrain first seen at 1:04), which I posted with the bug report:
Well, it’s not seen that well in the video and not that much like lava (but the closest thing the game has/had), but it was “glowing” (changing between different shades of dark red). The edges were also animated (seen when looking closely at 1:18/1:19).
There was an undocumented cool effect when painting Water on the Corruption terrain in Layering Mode, the former would regularly “descend” into the lava and resurface.
Why is it hidden? Well, the Battle Royale Trade Workshop is also hidden for some reason, even though one can just generate a BR map and copy it. The terrain might be hidden because it doesn’t work like in BR when used in a scenario, it neither expands nor swallows units.
Anyway, it would be cool if the expanding “Corruption” could be actived in a scenario via trigger, could be used for fantasy-themed scenarios.