Hello
Thank you for using Apple Support Communities.
It appears that you are getting a message that after iOS 14 you are getting a message that your flash is not usable until the iPhone cools down. We also see that you have done some troubleshooting already, resetting, erasing and updating to iOS 14.0.1. Just to be sure, you have made sure that the iPhone has been allowed to cool down, correct?
Take a look at the following support article for a few more steps to try:
If the camera or flash on your iPhone, iPad, or iPod touch isn't working - Apple Support
If you continue to have the same issues, then you would want to contact Apple Support directly for more specific assistance.
Best regards.