[Tactics]

> Nobody knew what happened ...

Yuki and Tengen told it to Itadori, Yuta and Choso that, after 0, Gojo used his fortune to purchase all curse nullification items (such as ISOH, Black Rope) on black market, halt it production and secure it somewhere (or destroyed it, they don't know) that Yuta recalled him and Miguel can't get any Black Ropes even when they asked elders that produced it.

While it possible he brings ISOH to fight, the best it could do is disable Mahoraga and that still questionable by risk and reward.
The risk? Backfired by either Sukuna get his hand on it or might as well he copied ISOH anti-CE flow properties; ISOH is cursed tool after all.
Man recognized Gojo CT recovery is possible by forced recovery part of brain in a minutes and use it as well, used high-output fire arrow after looking at several Jogo attack, copied Kenjaku cursed item production to change host; part of his fighting style alongside Shrine, no reason he won't do the same to ISOH.

By technicalities, saving Megumi is not within ISOH benefit as it doesn't fix Megumi coma state after Sukuna made him tank UV.