GMC Acadia vehicle (ATT) wifi is now “weak security” per Apple recent IOS upgrade!! Is this true?

You need to check the router settings - your network probably always had weak security, you just did not know it. iOS now warns you. That means the router needs to have its security settings adjusted or upgraded (or a new router should be installed). If this were your own router at home, you could easily do that by going to the router's website and making an adjustment or contact your internet provider and ask about a newer modem/router with more uptodate security settings. However, since you said that this is in your vehicle, you will need to ask the GMC provider about it. You would want a minimum of WPA2 (older) or the newest WPA3 setting.

I got the same notice with iOS 15 on my home router; I checked with my internet/Wifi provider, and we made the appropriate settings changes for it online.

Apple has nothing to do with vehicles' router or WiFi settings - they need to keep up with the latest technology. And, unless you are sending sensitive info via your vehicle Wifi to your employer, you can still use it just like you always have. As I said, those older settings have been classified as "weak" security as soon as the new protocol (WPA3) came out.