Non stun slaughter is legal in the UK for the consumption of the Jewish and Muslim faiths.
About 80% halal is stunned, none of shechita is.
A recent development is the reversible stun, one that renders the animal unconscious but if the animal isn’t bled can recover. This is being use in New Zealand for all of their Halal meat export.
A key part of both faith’s rules on slaughter is that death has to be by bleeding and the animal must be undamaged. As a captive bolt damages the skull its use cannot be accepted. However a head only electrical stun causes no damage so has been accepted many.
Historically a swift, massive severance of the carotid and jugular vessels was probably more humane than poleaxe and bleed. Percussive and electrical stuns should now be used in all cases.
A concern with kosher rules is that the hind quarters must have the sciatic nerve removed, the Gid Hannah. This increases the cost, and is sold into the general market, which is not right.
A concern with some stunning is that it is not very good. Most pigs are gassed by Carbon Dioxide, which is unpleasant and has been recommended to be banned. Many chickens are not stunned properly. The common factor being the profit motive and line speed.