Alt Text Creator uses the OpenAI o API to generate alternate text, which is subject to the OpenAI privacy policy. OpenAI says images are deleted after they are processed.
The Alt Text Creator extension does not collect or share any user data, except sending the API key and URL for a given image to OpenAI to create the alternative text. Your API key is not shared with anyone except OpenAI, but if you have cloud syncronization enabled in your web browser (e.g. syncing Google Chrome with a Google account), your API key is syncronized along with other browser data. You can delete API keys from the OpenAI website if your device or web browser syncronization has been compromised.