Stewing vegetables with the lid on is non-toxic, but it will spoil.

Once the lid is placed on the brine pot, when the water vapor evaporates upward and encounters the pot lid, it will condense into water droplets. Water droplets dripping along the pot lid and into the brine will destroy the integrity of the sealing oil, and the result will be spoilage.

If you cook stewed vegetables with the lid on, you will find that the meat has been stewed when you open the lid, and you need to clean up the brine and start over. Therefore, it is completely unnecessary to cover the stewed vegetables with a lid.

Extended information:

Braised food poisoning is mainly caused by nitrite. As a food additive, nitrite can react chemically with meat, making cooked food bright red and attractive in color. It can also inhibit the growth of Clostridium and Clostridium botulinum, thus extending the storage time of food.

The toxicity of nitrite is that it oxidizes normal hemoglobin into methemoglobin, thereby losing its oxygen-carrying capacity and causing hypoxia in the human body. But usually, the human body's powerful biochemical protection mechanism can metabolize methemoglobin that appears under physiological conditions.

Poisoning from eating braised meat is probably caused by the intake of too much food, which exceeds the metabolic capacity. As long as you control the amount of food you eat, you don’t have to worry too much.

People's Daily Online-Why braised cooked food makes people poisonous