I recently found this 1.58-bit model for ASR and it's surprising good (and very fast), that being said it's not a LLM.

https://huggingface.co/moondream/parakeet-redux

I don't think it's trying to be a useful implementation, but the significance they do provide is that they are able to improve on the relative loss at lower quants.

So, not something anyone would want to run currently, but an indicator that there is still more to squeeze out of lower precisions.

Trellis quantization is a far more approachable enhancement right now, but it doesn't cross the 1-bit barrier (and perhaps doesn't intend to).