This is not my original insight; it's something that comes up in the ACKS discord occasionally, but that I don't think I've ever seen talked about in the rest of the OSRosphere
It can seem kind of cruel to have monsters with save-or-die poison on the first level of the dungeon. But it's actually important for the long haul.
Save-or-die poison threatens characters regardless of their level (particularly in combination with "a natural 20 always hits"). When you put save-or-die monsters in the first level of the dungeon (and then all the rest) you are saying "it doesn't matter how high level you are, adventuring anywhere always carries some risk, no matter how small, of death."
There is, in effect, a floor on the amount of risk that you are allowed to assume while still calling your activity "adventuring". Without some amount of real risk, would it be an adventure? No.
I think the existence of this risk floor is important to campaign gameplay as well as semantics. Making adventuring never quite safe discourages players from just sticking to the weakest areas and grinding out levels in as much safety as they can manage, because "as safe as you can manage" still always has "losing the game" on the table. Since you must take risks, you probably want to assume slightly more risk for a much higher payout (ie, push into deeper levels of the dungeon). So it helps keep the campaign from entering a degenerate state and helps motivate progress through the environment. It's probably also important for keeping play with mixed-level parties from getting boring for the players with higher-level characters. Finally, in-world, it also helps explain why high-level adventurers might want to hire low-level adventurers for certain jobs / delegate; the risk to those high-level adventurers is low, but never zero. Better to let someone else do the dying if you can get them to.