It does? I just tested with "!g test" and got redirected to https://www.google.com/search?q=test as expected. Not that I ever really use that bang any more given how bad Google’s results have become.

It does work. Your problem may be different, but it temporarily stopped working for me because I set "Search Engine to Redirect" in the Kagi Extension to Google (because my browser search engine default was Google). My "!g" searches ended up being redirected back to Kagi. It started working again when I changed the browser default and extension to something different.

I thought you can add your own bangs as well.

bang first or last works both for me.