The short version is that VG Poison Types have never been their own TCG Type, and likely never will be because the powers-that-be struggle with structuring the game for... well... really even the original seven TCG Pokémon Types, let alone the 11 we'd had before Sword & Shield. Nothing is a perfect fit for Poison, but Darkness seems to fit it as well or better than Psychic, both in terms of aesthetic and game mechanics.
Of course, to me the real question is why didn't they move the VG Rock Type from TCG Fighting to TCG Metal, and why - if they were going to cut the Fairy Type anyway - didn't they merge them into the TCG Dragon Type instead of keeping the TCG Psychic Type overstuffed.