It's the fact that the site does not have the new EU security enhancements. It is safe, just not up to the new standards. This is why in chrome you get the "not secure" warning. It IS in fact secure, just not as secure as the browser wants it to be.
In order to fix this, I would have to create an entire new Forum with the new enhancements then restore a backup from this one to the new one. There is no guarantee this would even work successfully. Frankly, it's fine how it is. I don't ask for any kind of financial or sensitive information so you're good.
Thanks for asking.