The safety of the chicken in your slow cooker depends on the internal temperature it reached during the two and a half hours it was on "Warm" mode and how quickly it is brought up to a safe temperature.
To ensure food safety, chicken (and other poultry) should be cooked to an internal temperature of at least 165°F (74°C) to kill any harmful bacteria that may be present. If the chicken was in the "Warm" mode, it is possible that it did not reach this safe internal temperature, especially if it was initially raw or partially cooked.
If the chicken has been sitting in the slow cooker on "Warm" for two and a half hours and didn't reach the safe internal temperature of 165°F, there is an increased risk of harmful bacteria growth. Consuming undercooked chicken can lead to foodborne illnesses.
To salvage the situation, I recommend the following steps:
Immediately switch the slow cooker to "High" mode and continue cooking until the chicken reaches an internal temperature of 165°F (74°C).
Use a food thermometer to check the temperature of the chicken in multiple spots to ensure it's evenly cooked.
If the chicken doesn't reach 165°F within a reasonable time (within an hour or so), it's best to discard it to avoid any potential health risks.
In the future, it's crucial to follow the recommended cooking times and temperatures when using a slow cooker to ensure food safety. Always cook chicken until it reaches the safe internal temperature of 165°F, and if you need to keep it warm after cooking, use the "Warm" mode briefly before serving rather than relying on it as a cooking method.