It didn’t make it in, but someone is investigating it
Waikuteru has been doing fantastic work in the Zelda: Breath of the Wild sphere. It’s like every few months they discover some cool scrapped mechanic, or they do a larger dive into an existing concept. This time they’re looking at poison water (which drains health): a fascinating relic that was removed from the game, but put back in thanks to mods.
If you touch the water, it drains your health. Enemies also suffer damage from being in it, which would have created all sorts of wacky trap situations in the real game had it existed. Waikuteru even sets up a trap for an NPC to walk in, and witnesses a Hinox bathing in the poison water (which slowly takes damage from it, then abruptly dies).
As Waikuteru points out, you can’t use Cryonis on the poison water by default. Swimming physics do work as normal, and you can use a raft to sail on through it. To get this to work, the creator poisoned “select spots” through modding, so this technique can be used to craft all sorts of hazards in a custom version of the game.
As the video posits, there are a lot of developer-made artifacts with the poison water, hinting that it was likely near finished, but scrapped. It easily could have been limited to just a few perilous zones, but maybe they ran into some sort of technical issue and decided to abandon it altogether (the way the Hinox reacts to it hints at this). Or, maybe it’ll come back in the sequel!