If it’s water based, the temperature won’t go over 100 C. Ideally, you’d want to simmer it below that rather than cook it at a high boil. Then you’d just need to make sure there was enough water in it that it wouldn’t all evaporate off while unattended (though more accurately, you’d want enough water to prevent the bottom part from drying out faster than more water can replace it to avoid it burning on the bottom, though that’s not so much a safety issue as it is a quality issue). Or just cover it so that any evaporating water recondenses and ends up back in the stew (though this only really slows the rate at which you lose water, since the pressure buildup will force the cover open and let some steam escape and many covers have a hole to equalize the pressure, so still keep an eye on water levels if you do a long cook).

If all the water evaporates, then the heat can rise, potentially to a flash point of some ingredient, which would start a fire, which I’d think would be the main safety issue with a slow cook like that, assuming you maintain a safe temperature above 60 C.

For microbial food safety, cooking over long periods is safer than soaking, generally speaking. It depends on how it is prepared/stored.

Like canning or jarring could be considered a soak, but you need to seal the container (so no new microbes get in) and cook it in the jar (to kill off any microbes that were already on the food), or use another method that creates an environment hostile to microbes, like make it too salty or acidic.

Or another option is to deliberately introduce microbes that play nice with our guts and allow it to ferment, which is essentially allowing it to digest a bit outside of our guts. The idea there is that any new microbes that try to move in can’t compete with the existing colony and either die off or maintain a population small enough to not cause harm.

A long cook is basically maintaining the temperature that canning uses to kill off microbes without then sealing it away from new ones. New ones will arrive but then die due to the heat.

Note that some foods can break down into harmful compounds if cooked long enough or can contain harmful compounds that require a boil to cook off, like kidney beans. Also if the food already contains heat-resistant toxins, obviously cooking it for a long time won’t get rid of them.

My family in Jamaica make their goat stew overnight. Just leave the fire going. Safe? Probably not, but very widely practiced.

Low and slow? Ever had BBQ? If that shit wasn’t cooked overnight, miss me with that shit. (Unless it’s turkey or chicken obviously).