I use Apple TV 4th gen. I use the Home software and the Apple TV as a hub. I was told to upgrade my Home to the latest software. Now the Apple TV no longer works. I have lost integration to my other home systems. What can I do?

If you Upgraded to the new Home architecture any device that is connected to an upgraded home and not running the latest software will lose access to the home until the device is updated. This means you can’t use the Home App to control Homekit

You cannot downgrade. The upgrade was optional and you might not have noticed it contained a warning about this. The only solution is if you can update your MacOS to Ventura MacOS for you to use the Mac as before. If you have an iPhone or iPad running OS 16 then you can still use the Home App on those. Your Apple TV should still be acting as a Hub, so long as you’ve updated it to tvOS 16.5 with it signed into HomeKit.