CO2 is common in labs though lab rats/mice do show aversion to CO2 which is interpreted to mean that it causes distress. So there's been ongoing debate for 20+ years over how humane CO2 is and what the correct flow rate should be and if oxygen should be added (or not) or other gasses (or not).[1]

Something physical like direct concussion/blunt force trauma followed up by another method might be better[2] (needs more studying) but there's also issues with physical methods because they require more skill to do correctly (or tools/machines developed to negate the skill factor).

Also operators dislike physical methods compared to gassing[1] so what's most practical might end up the standard[3] even if it may not be the most humane.

I mean, theoretically if you could instantly crush a mouse, faster than brain/nerve signals can travel, that would be humane, right? But then a human has to deal with a gruesome crushed mouse.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5035945/

[2] https://journals.sagepub.com/doi/full/10.1177/00236772221097...

[3] https://www.avma.org/sites/default/files/2020-02/Guidelines-...