The message indicates some part of the webpage, or the entire webpage, is using an insecure (HTTP) connection, not a secure (HTTPS) connection.
Webpages can be complex and with parts loaded from many different servers, but some part of all that is using HTTP and not HTTPS.
It is preferable that all parts of a webpage use HTTPS, as otherwise sensitive or private data or passwords can be exposed.
Here is how to disable the warning: Change Security settings in Safari on Mac - Apple Support
The preferred solution is for the webpage maintainer to fix the webpage.
This is not related to an expired root certificate in Keychain.
macOS 13.7.4 is current.