Why Safari is telling me that a website is unsafe?

And maybe that site is using antiquated security protocols (e.g. TLS 1.0, 1.1) that Safari is attempting to alert you about. Enter that website URL in the following browser form, and it will provide a report on the site's security status.

SSL Server Test (Powered by Qualys SSL Labs)

GIve it a little time to provide its summary and overall rating at the top of the browser window. It will tell you what version of TLS is being used, and other information.

Maybe the website's security cert has expired or is not properly configured. Maybe you have installed some third party app that is falsely providing warnings. But these are only a guesses since you have not provided any notification, system, or site details.

Outstanding site and have bookmarked 👍