Leaving chili (or any food) in a slow cooker without a lid on high heat overnight is not recommended and can pose a potential food safety risk. The slow cooker relies on the lid to trap heat and maintain a consistent temperature for safe cooking. Without the lid, the food may not reach and maintain a high enough temperature to prevent the growth of harmful bacteria.
Bacteria can multiply rapidly in the "danger zone" temperature range of 40°F to 140°F (4°C to 60°C). When food sits within this temperature range for an extended period, it can become unsafe to eat. Overnight is far too long for food to be exposed to these conditions.
As a general rule, perishable foods like meat, poultry, and dairy products should not be left at room temperature for more than two hours. If the ambient room temperature is above 90°F (32°C), this time decreases to one hour.
Given the circumstances, I would advise against consuming the chili that was left in the slow cooker without a lid on high heat overnight. It's better to prioritize food safety and discard the chili to avoid the risk of foodborne illness. Always make sure to cook and store food properly to prevent any potential health hazards.