Leaving the ladle in the soup considered harmful?

While I agree with john2103 I would also add that if the soup is thickened with tapioca starch, the soup could un-thicken due to the possible moving ladle.

If the ladle was made of aluminium (or aluminum, if you prefer), then it would be a bad idea to leave it in anything acidic, as it could contaminate the food with aluminium salts, which may (it's controversial) be implicated in Alzheimer's.

If you had a plastic ladle, it may not be able to handle high-heat for an extended period of time.

If it were in contact with the bottom of the pot, it's possible that it would get above 100°C, and depending on the material, could soften. It probably wouldn't melt entirely, but it'd be shocking enough that you'd question if anything leached into the soup, and likely want to pitch the whole batch.

Most higher-end kitchen utensils these days are made from silicone, you're less likely to have the problem, unless you're buying discount utensils (eg, shopping at the dollar store, or the random kitchewares tent at some flea markets) or have older stuff (either inherited or from yard sales).

Someone once told me that leaving a metal spoon in a metal pot containing food is bad because of electrolysis. I've searched for confirmation online but haven't found any.