Common pitfalls of the key is it may be accidentally shared by posted code on GitHub or social media.
If anyone gains access to your key, they will have access to your Adafruit IO account.
Your key can be regenerated within IO by clicking the yellow AIO Key text in the header of the page.
Once you click that link, a dialog box will show which will allow you to click REGENERATE AIO KEY.
This process is not reversible.
For further reading on IoT security, we have an entire guide that explains various types of attacks and how to protect yourself.