How can weak security message be removed when not true?

The weak security message you are receiving is based on the wi-fi security you are using on your wi-fi network. It had always been true, it is just that Apple has provided the information to let you know now. See this for information on recommended settings for your wi-fi to ensure that you are using the best security settings. Recommended settings for Wi-Fi routers and access points - Apple Support

It sounds like you are not connecting to the correct network. I suggest that you forget your network, restart the phone, then reconnect to your wi-fi. It will only display the message if you are not using the secure wi-fi.